Gears & Tires

[Timmmmmy]

So I will be picking up tires and re-gearing pretty quickly. I am hoping to get some opinions on the right setup for my needs.

 

Truck: 4.0, Manual, 4X4, 4.5" lift up front, 5.5 in the back, 3.07 gears, D44 in the rear

 

Use: Daily driver in semi hilly area, camping and wheeling requires an hour on the freeway, needs to cruise comfortably at 120/KPH or 75/MPH. Trails are very steep and usually wet. I really don't wheel that hard, just getting into it.

 

I had settled on 4.10 gears and 265/75/16 (31.9 - 10.5") BFG KM2's (Have ring/pinion/carrier etc for my D30 already)

 

I am having second thoughts for a few reasons:

 

1.) My mechanic and other people have said this might be too low of gearing for highway driving, too slow

 

2.) Wheeling buddies have said to just go with 33X11.5

 

The reasons I am considering the 31.5's is because they should fit my lift without fender flares and wheel spacers and because I assume it will help reduce fuel consumption

 

So I guess I'm wondering if:

 

1.) 4.10's and 31.5's will be to low for highway use

 

2.) Whether I should just go with 33's and put cut out fender flares and or 6" coils in. Will this really increase fuel use that much?

 

Any advice is appreciated, I just want to make sure I get it right the first time, can't afford to do it twice, Ryan

[87Warrior]

My TJ Rubi (4.0/5spd) came with 4.10s and ~30" tires from the factory. It was a perfect combination with plenty of power.

 

I found the 285/75/16 (about 33x11.5") on the same Jeep to be decent for a daily driver/weekend wheeler. 

 

Smaller tires are lighter = less rolling resistance = better fuel economy (generally). I would not go with a bigger tire simply to go bigger. Not to mention, 6" of lift requires more than coils, especially on a daily driver.  It doesn't sound like you really need bigger tires to meet your wheeling needs. 

[TheDirtyJeep401]

The bigger the tires the more it takes to move them, depending on how you have it geared.

[Eagle]

he reasons I am considering the 31.5's is because they should fit my lift without fender flares and wheel spacers and because I assume it will help reduce fuel consumption

 

So I guess I'm wondering if:

 

1.) 4.10's and 31.5's will be to low for highway use

 

2.) Whether I should just go with 33's and put cut out fender flares and or 6" coils in. Will this really increase fuel use that much?

 

Any advice is appreciated, I just want to make sure I get it right the first time, can't afford to do it twice, Ryan

 

In terms of revolutions per mile (which is the only important criterion when matching gears to tire size), the 265/75-16s are identical to 33x12.50-15s (637 revolutions per mile vs. 636 revolutions per mile).

 

Using that as the base, my speed to RPM chart (which is corrected for actual revolutions per mile, not theoretical tire diameter that doesn't accound for sidewall squat), I get the following (in 5th gear):

 

4.10 gears ==> 2282 RPM at 70 MPH (my chart doesn't go up to 75 MPH)

 

4.56 gears ==> 2538 RPM at 70 MPH

 

4.88 gears ==> 2716 RPM at 70 MPH

 

Crunching the numbers, to convert that to 75 MPH we get:

 

4.10 gears ==> 2445 RPM at 75 MPH

 

4.56 gears ==> 2719 RPM at 75 MPH

 

4.88 gears ==> 2910 RPM at 75 MPH

 

You should be changing to 4.88 gears. 75 MPH cruise at 2900 RPM is NOT pushing the engine too fast. Conversely, 4.10 gears isn't nearly enough gear for those tires. Remember, a "31x10.50" tire is NOT 31 inches in diameter -- they are typically between 30.3 and 30.5" overall diameter. So if you're thinking that your chosen tires are close to 31s ... you're wrong.

 

If you're not comfortable with 4.88s, go with at least 4.56 gears.

 

Putting this is perspective, a stock XJ with automatic and 225/75-15s turns 2261 RPM at 70 MPH, so if you go with 4.10s you're getting exactly the same final drive ratio as a stock vehicle ... but you're trying to spin much larger and heavier tires.

[Timmmmmy]

My TJ Rubi (4.0/5spd) came with 4.10s and ~30" tires from the factory. It was a perfect combination with plenty of power.

 

I found the 285/75/16 (about 33x11.5") on the same Jeep to be decent for a daily driver/weekend wheeler. 

 

Smaller tires are lighter = less rolling resistance = better fuel economy (generally). I would not go with a bigger tire simply to go bigger. Not to mention, 6" of lift requires more than coils, especially on a daily driver.  It doesn't sound like you really need bigger tires to meet your wheeling needs. 

 

Yeah thats what I thought. I remember hearing that 31's and 4.10 was a good  combo. I was just worried that it would be great for wheeling but a dog on the highway. One of the guys at work who is a huge car guy just re-geared his durango to 4.10's and said its pretty slow now but the acceleration is good. I think 31.5s are fine for me at the moment as well. I would be going bigger for the sake of going bigger. Anyway thanks for the input. Very much appreciated!

[Timmmmmy]

 

he reasons I am considering the 31.5's is because they should fit my lift without fender flares and wheel spacers and because I assume it will help reduce fuel consumption

 

So I guess I'm wondering if:

 

1.) 4.10's and 31.5's will be to low for highway use

 

2.) Whether I should just go with 33's and put cut out fender flares and or 6" coils in. Will this really increase fuel use that much?

 

Any advice is appreciated, I just want to make sure I get it right the first time, can't afford to do it twice, Ryan

 

In terms of revolutions per mile (which is the only important criterion when matching gears to tire size), the 265/75-16s are identical to 33x12.50-15s (637 revolutions per mile vs. 636 revolutions per mile).

 

Using that as the base, my speed to RPM chart (which is corrected for actual revolutions per mile, not theoretical tire diameter that doesn't accound for sidewall squat), I get the following (in 5th gear):

 

4.10 gears ==> 2282 RPM at 70 MPH (my chart doesn't go up to 75 MPH)

 

4.56 gears ==> 2538 RPM at 70 MPH

 

4.88 gears ==> 2716 RPM at 70 MPH

 

Crunching the numbers, to convert that to 75 MPH we get:

 

4.10 gears ==> 2445 RPM at 75 MPH

 

4.56 gears ==> 2719 RPM at 75 MPH

 

4.88 gears ==> 2910 RPM at 75 MPH

 

You should be changing to 4.88 gears. 75 MPH cruise at 2900 RPM is NOT pushing the engine too fast. Conversely, 4.10 gears isn't nearly enough gear for those tires. Remember, a "31x10.50" tire is NOT 31 inches in diameter -- they are typically between 30.3 and 30.5" overall diameter. So if you're thinking that your chosen tires are close to 31s ... you're wrong.

 

If you're not comfortable with 4.88s, go with at least 4.56 gears.

 

Putting this is perspective, a stock XJ with automatic and 225/75-15s turns 2261 RPM at 70 MPH, so if you go with 4.10s you're getting exactly the same final drive ratio as a stock vehicle ... but you're trying to spin much larger and heavier tires.

Hmmmm. well you certainly have given me a lot to think about.

 

I have heard the KM2's are a true tire size compared to other makes but that could very well be heresay. From what I have read the KM2 265/75/16 is 31.9" but on a 8" wide rim it sits at 31.5. At any rate the RPM breakdown is very helpfull. I was thinking running the engine at close to 3000 rpm was on the high side as 2250 was the sweet spot for the 4.0.

 

I have some thinking to do, your input is much appreciated Eagle!

[Stacks]

My experience has been that the sweet spot changes with each tire and gear combo. My stock TJ with 31's and 3.07 liked 2400-2500 in 4th gear. 5th unuseable unless you wanted to cruise at about 88mph.  I averaged 22-24 mpg commuting to work. With 4.10's and 31's, 4" of lift it likes 2700 and gets around 19mpg. Again in 4th. 5th lugs it too much.  Vacuum drops to 7-8" at 2700 where in 4th I run about 12-13" at 2700.  I agree with Eagle, 3000rpm isn't pushing the 4.0 at all.

[Timmmmmy]

Funny how I started this thread worrying 4.10's was to deep and now I'm worried they are not deep enough.

 

After looking at the chart I am starting to think 4.10's are to low, at least in principle. I get that if the top speed I am likley to cruise at is around 75/MPH then having the RPMS around 2500 - 2900 is alright. I really like the idea of being able to get into fifth earlier and actually use the overdrive. I do have a few questions though.

 

1.) I have heard that the ideal RPM for fuel effeciency is around 2250 on the 4.0L. If I'm cruising on the freeway at 2600 - 2900 RPM wouldn't I be getting poor fuel millage? I understand its a Jeep and you don't drive one for economy but it does impact how much I can take it out and how far I can go. I would like to keep fuel costs as reasonable as possible.

 

2.) I know it has been said that running even as high as 2900 RPM isn't bad for the engine but I can't help think that driving at that level for extended periods must put additional ware on the engine. Keep in mind I travel over an hour at that speed on the freeway. Several times a summer I head North and could be running at those revs for four plus hours straight. Can I be sure this isn't having an impact on the engine?

 

3.) Forgive my ignorance but this is my first manual vehicle and I have only driven on 31's with 3.07 gears. Would driving with this low of gearing (4.56) be comfortable as a daily driver? I guess I'm thinking of having to start in second or it just being awkward to drive in town. I'm worried I won't like how it drives.

 

At this point I am thinking of going with 4.56 gears on the 265/75/16 BFG KM2's I have picked out. I am pretty nervous though. If anyone has any experience of daily driving with a similar setup or can answer any of my concerns it would be very much appreciated. Ryan

[Bmoore2]

I have 4.56 gears in mine with 33 x 12.50. I have only drove it about 40 miles since the swap. But the seem to be spot on for me. :thumbsup:  Give me till after this weekend and I can give you some more info after I drive it some more. The 3.07 gears that were in mine were horrible with the larger tires. HUGE DIFFERENCE and I can use 5th gear now. Before the gear swap I never even used 5th gear.

[TheDirtyJeep401]

I have 4.56 gears in mine with 33 x 12.50. I have only drove it about 40 miles since the swap. But the seem to be spot on for me. :thumbsup:  Give me till after this weekend and I can give you some more info after I drive it some more. The 3.07 gears that were in mine were horrible with the larger tires. HUGE DIFFERENCE and I can use 5th gear now. Before the gear swap I never even used 5th gear.

Out of curosity, whats your MPG now?

[Eagle]

I have heard the KM2's are a true tire size compared to other makes but that could very well be heresay. From what I have read the KM2 265/75/16 is 31.9" but on a 8" wide rim it sits at 31.5. At any rate the RPM breakdown is very helpfull. I was thinking running the engine at close to 3000 rpm was on the high side as 2250 was the sweet spot for the 4.0.

 

I have some thinking to do, your input is much appreciated Eagle!

 

Somewhere around 2250 to 2500 is a sweet spot for normal driving, but if you set up your gears to run those RPMs at 75 MPH then at anything under 60 MPH 5th gear will be useless, and 4th gear will be almost useless. Also don't forget that drag increases exponentially with speed, so to push the same "brick on wheels" down the road at 75 MPH requires a whole bunch more horsepower than pushing the same brick on wheels at 50 MPH. 2250 is the torque peak, but it isn't the horsepower peak.

 

What you're shooting for would be almost exactly the same as stock 3.08 gears and stock tires. That setup runs a bit over 2100 RPM at 75 MPH, and 5th gear is totally useless under 60. Not only that, I have run the same route, one way using 5th gear and the other way using only 4th, and I got as good or better gas mileage NOT using 5th gear. The 3.08 ratio was an idiotic choice by AMC/Jeep, but they did it because the fuel mileage and emissions testing was run indoors on a dynomometer, so there was no aerodynamic drag to be concerned with. In other words, they weren't realistic numbers, and the gear ratio was chosen to beat the test, not because it was in any way a good choice for real world driving. So what you would be doing trying to run 2250 RPM at 75 MPH is to duplicate the stock 5-speed overall drive ration, while struggling to spin much larger and heavier tires.

 

Keep in mind that the 4.0L engine (and the 2.5L) is derived from AMCs old I-6 that was first introduced all the way back in 1964. Back then it was carbureted, of course, but the basic block and engine was the same. And in those days we didn't have overdrive, a manual transmission was "three on the tree." Top gear was third, with a 1:1 ratio in the transmission and 3.08 gears in the axle. In my 1966 Rambler American, that worked out to exactly 2500 RPM at 60 MPH, and 3000 RPM at 72 MPH. They ran forever like that. Mine got 28 MPG highway and mid-20s around town. I was in the Army then and I traded mine in for a new Javelin with my combat pay. My brother had a 1970 Gremlin with the same drive train that went something like 350,000 miles. And he autocrossed it on the weekends.

 

If you set yours up to cruise at 3000 RPM at 75 MPH, you won't be "burning up" the engine, and you'll have a much happier rig.

[87Warrior]

If you set yours up to cruise at 3000 RPM at 75 MPH, you won't be "burning up" the engine, and you'll have a much happier rig.

I completely agree with this. My TJ purrs like a kitten at 75mph at 3k RPM. It is set up with 5.13 gears and ~35" tire.

 

That said, I noticed a drop in highway fuel economy when I went with the 5.13's. With 35's and 4.10's I could muster 16mpg on the highway with cruise set at 75 (actual speed) but required a lot of down shifting. Now with the 5.13's I get about 14mpg with cruise set on 75 (actual speed) and I can leave it in 5th on the interstate. In town with the 4.10's I averaged14mpg and now with the 5.13's I average 17mpg. Seems crazy, but I have reached these numbers at every fill up for the last 4 years with the 5.13's.

[Pete M]

I've got 4.10s in the 88 (4.0/stick) and I'd kill for some 4.56s.  the driving is certainly better with my 31s than my 33s, but I'd never pay for 4.10s.  that was the ratio that was already in the ford axles.  If I was paying all that money for a re-gear, I'd go no less than 4.56.  Some pathfinders/rodeos came with 4.30 gears and tires that were way less than 31".  Heck, my Dakota has 3.92 gears and the tires are only like 28" tall.  It's a myth that Jeeps struggle on the freeway with gears that are just a bit deeper than stock.

[mvusse]

4 cylinder, 28.9" tires, AX4, 3.55 axles.

75mph in 4th gear was running 3100 rpm and got me 19mpg.

 

Threw in an AX5, keeping everything the same.

Now 75mpg in 5th gear is running 2650 rpm and gets me 24 mpg.

 

I must concede that 5th gear is absolutely gutless, though, and useless on any grade steeper than 1 or 2%.

[Timmmmmy]

Eagle, thanks for posting all that, it really helped me to wrap my head around what I was missing

 

One thing though. You suggested the 4.88's based on the 265/75/16 tires having the same revolutions per mile as a 33X12.5X15. From my conversions this is a 31.6" (Most likely smaller than advertised) and I should go off the 31.5X10.5 table on your excel chart. Going off your chart with 4.56 at 70MPH I would be at 2713 in fifth gear, I imagine I would be around 3000 RPM at 75/MPH. This seems to be the best gearing option. Does this sound correct?

 

Also looking at the 31X10.5 chart I notice that say with 4.56 at 70/MPH in 5th gear I would be at 2713/RPM. The 1:1 for this tire size which I think represents 4th gear would be 3618/RPM. As 5th gear is not good for climbing hills, would keeping up high top speed be a problem when climbing hills? Having to climb hills in fourth seems like I would be either going pretty slow or revving very high, is this a correct assumption?

 

It isn't a huge concern as most of my freeway driving is flatish but when I head North I take a very hilly highway. If you have ever seen that highway to hell reality show this is the HWY, the Coquihalla Pass, it has some very long and steep grades. Should I be worried about keeping speed up on hills?

 

Also I havn't calculated my MPG. I'll try to work it out next fill

[FrankTheDog]

I ran my 98 XJ for 180,000 miles on 265/75/16's with 4.10 gears.

There is nothing wrong with that ratio. I've towed with it and have driven many highway miles.

MPG was from 18-20 average to 22 mpg running down I-81 to tennesee.

On the way back from tennesee I dragged a Volkswagen camper bus that weighed 4300 lbs plus the weight of a dolly and averaged 15.

Went 80 mph most of the way down and 50 on the way back.

80mph was 3,000 rpm. That jeep has 400k on it now with the original motor.

[Eagle]

 

Eagle, thanks for posting all that, it really helped me to wrap my head around what I was missing

 

One thing though. You suggested the 4.88's based on the 265/75/16 tires having the same revolutions per mile as a 33X12.5X15. From my conversions this is a 31.6" (Most likely smaller than advertised) and I should go off the 31.5X10.5 table on your excel chart. Going off your chart with 4.56 at 70MPH I would be at 2713 in fifth gear, I imagine I would be around 3000 RPM at 75/MPH. This seems to be the best gearing option. Does this sound correct?

 

Also looking at the 31X10.5 chart I notice that say with 4.56 at 70/MPH in 5th gear I would be at 2713/RPM. The 1:1 for this tire size which I think represents 4th gear would be 3618/RPM. As 5th gear is not good for climbing hills, would keeping up high top speed be a problem when climbing hills? Having to climb hills in fourth seems like I would be either going pretty slow or revving very high, is this a correct assumption?

 

What chart are you using?

 

The problem with most charts, and with most calculations, is that they are based on the tire diameter as it sits on a rim, not mounted on a vehicle. Once you put it on a vehicle, supporting weight, the actual static radius from the axle to the ground is less (often significantly less) than half the theoretical diameter. When I made up my spreadsheet, I went to tire company brochures and web sites and found their own numbers for actual revolutions-per-mile. Even that's an approximation -- within any nominal size, one company's 31x10.50-15 may have a few different revolutions-per-mile than another company's 31x10.50-15. But it's still a lot more accurate than just calculating a diameter from the nominal tire size and dividing it by two.

 

Look up 31x10.50-15 from any tire company and I think you'll find it isn't 31 inches in diameter. Most seem to run about 30-1/2".

 

 

Also looking at the 31X10.5 chart I notice that say with 4.56 at 70/MPH in 5th gear I would be at 2713/RPM. The 1:1 for this tire size which I think represents 4th gear would be 3618/RPM. As 5th gear is not good for climbing hills, would keeping up high top speed be a problem when climbing hills? Having to climb hills in fourth seems like I would be either going pretty slow or revving very high, is this a correct assumption?

 

31x10.50s with 4.56 gears is LESS overall gearing than a stock 4.0L with automatic and stock 3.54 gears. For the stock setup, 70 MPH in 4th (overdrive) is 2913 RPM and downshifting to third (1:1) bumps it to 3884 RPM. My wife's XJ is an automatic, and there's no problem climbing most hills on Interstates without downshifting. Any incline that's so steep you need to downshift for it probably won't be the end of the world if you only drive 65 instead of 75. But if you have the right ratio, you won't need to downshift as much on upgrades. This is why I think that factory's choice of 3.08 gears behind the 5-speed was so dumb. I have to downshift a lot even at 60 MPH on highways, and I should not have to do that.

[mvusse]

 

 

Eagle, thanks for posting all that, it really helped me to wrap my head around what I was missing

 

One thing though. You suggested the 4.88's based on the 265/75/16 tires having the same revolutions per mile as a 33X12.5X15. From my conversions this is a 31.6" (Most likely smaller than advertised) and I should go off the 31.5X10.5 table on your excel chart. Going off your chart with 4.56 at 70MPH I would be at 2713 in fifth gear, I imagine I would be around 3000 RPM at 75/MPH. This seems to be the best gearing option. Does this sound correct?

 

Also looking at the 31X10.5 chart I notice that say with 4.56 at 70/MPH in 5th gear I would be at 2713/RPM. The 1:1 for this tire size which I think represents 4th gear would be 3618/RPM. As 5th gear is not good for climbing hills, would keeping up high top speed be a problem when climbing hills? Having to climb hills in fourth seems like I would be either going pretty slow or revving very high, is this a correct assumption?

 

What chart are you using?

 

The problem with most charts, and with most calculations, is that they are based on the tire diameter as it sits on a rim, not mounted on a vehicle. Once you put it on a vehicle, supporting weight, the actual static radius from the axle to the ground is less (often significantly less) than half the theoretical diameter. When I made up my spreadsheet, I went to tire company brochures and web sites and found their own numbers for actual revolutions-per-mile. Even that's an approximation -- within any nominal size, one company's 31x10.50-15 may have a few different revolutions-per-mile than another company's 31x10.50-15. But it's still a lot more accurate than just calculating a diameter from the nominal tire size and dividing it by two.

 

Look up 31x10.50-15 from any tire company and I think you'll find it isn't 31 inches in diameter. Most seem to run about 30-1/2".

 

 

>Also looking at the 31X10.5 chart I notice that say with 4.56 at 70/MPH in 5th gear I would be at 2713/RPM. The 1:1 for this tire size which I think represents 4th gear would be 3618/RPM. As 5th gear is not good for climbing hills, would keeping up high top speed be a problem when climbing hills? Having to climb hills in fourth seems like I would be either going pretty slow or revving very high, is this a correct assumption?

 

31x10.50s with 4.56 gears is LESS overall gearing than a stock 4.0L with automatic and stock 3.54 gears. For the stock setup, 70 MPH in 4th (overdrive) is 2913 RPM and downshifting to third (1:1) bumps it to 3884 RPM. My wife's XJ is an automatic, and there's no problem climbing most hills on Interstates without downshifting. Any incline that's so steep you need to downshift for it probably won't be the end of the world if you only drive 65 instead of 75. But if you have the right ratio, you won't need to downshift as much on upgrades. This is why I think that factory's choice of 3.08 gears behind the 5-speed was so dumb. I have to downshift a lot even at 60 MPH on highways, and I should not have to do that.

 

 

I swapped my daughter's XJ over to 4.10 gears, and put 31x10.50s under it. Stock tire size on that one was 225/75R15 with 3.55 gears. That geared it quite a bit lower than stock. It is now running 265/75R16 (~32x10.50) and it is closer than with the 31s, but still off 3%. Not going with larger tires for a while, so will be getting a different speedo gear. 1 tooth more should make it pretty much dead on.

 

When 4.10s with 31x10.50s is already geared lower than stock, 4.56 would be *way* lower than stock.

 

With stock gears and stock tires going down I-77 at 70mph it ran about 2300 rpm, not anywhere near the 2900 you are claiming. 2900 is about right for third gear (1:1) on an AW4, or 4th gear with a 2.5/AX4 combo, which gets the same 3.55 axle ratio.

[Bmoore2]

 

I have 4.56 gears in mine with 33 x 12.50. I have only drove it about 40 miles since the swap. But the seem to be spot on for me. :thumbsup:  Give me till after this weekend and I can give you some more info after I drive it some more. The 3.07 gears that were in mine were horrible with the larger tires. HUGE DIFFERENCE and I can use 5th gear now. Before the gear swap I never even used 5th gear.

Out of curosity, whats your MPG now?

Not sure yet..... Only drove it about 40 miles home from the shop. I will let you know probably Monday, planning on driving it around this weekend. :thumbsup:

[Timmmmmy]

Eagle: I am going off your spreadsheet stickied in the DIY forum. Granted I am still trying to wrap my head around numbers but thats where I got it from. Also I have read that automatics need/benifit from lower gearing than manuals do. Is there any truth to this?

[Pete M]

quite the opposite.  automatics are a bandaid that lets you get by with less gearing (numerically lower).  Manuals need more gearing so they don't fry the clutch as easily when starting from a stop.

 

I really hate those charts.  way too complicated and you start with the numbers that Jeep supplied from the factory (which SUCK).  This is mine for when you're paying for gear installs (aka: spending insane amounts of money):

 

got a 2.5?  4.56 for 30/31s, 4.88 after that

 

got a 4.0?  4.10s for 30/31s, 4.56 fr 31s to 35s, 4.88s for 35+

 

these rules change if you're grabbing axles from the junkyard with gears already in them (aka: doing this on the cheap) since 4.10s are the most common deep gear for axles that are easy to swap in. 

[Bounty Hunter]

You don't take into account the large increase in tire mass?

 

In terms of revolutions per mile (which is the only important criterion when matching gears to tire size

[Timmmmmy]

Well I'm pretty sure the truck will be getting 265/75/16's and 4.56 gears. I'm feeling pretty comfortable with that setup. It is a shame as I just bought a good used D30 with 4.10's from a guy on my local jeep club board. Fresh gears front and rear will be pricy and it may have to wait a little longer but I want to do it correctly.

 

I appreciate all the help and input. The tires/rims/gears will be more than I paid for truck and that was a big investment. I know the gears/tires question has been asked many times and beleive it or not I have read as many of the threads as I could find. I was still unsure though, largly regarding the drivability of gearing from 3.07 to 4.56/4.88's. So thanks again!

 

Ryan

[Biotex]

I was waiting until you got the info you needed before I jumped in with my comment. Didn't want to hijack, but do want to give my point of view. I have a healthy 4.0L in Maura, and just a couple of days ago I installed some used 31's on steel rims in place of my stock tires on aluminum rims. Gearing is the original 3.07's. I drove 200 miles of mixed terrain, and so I wanted to see the differences so I could decide what gearing i want.

 

With the 31's, I'm easily able to reach 85 mph and cruise there. Takes a lot more pedal, but doable. Not sure about the fuel mileage, but the rpms were right around 2000 at 80mph.

 

What I found, was that if I keep in 4th gear, and cruise at 65 or 70mph, everything is good. Yet 5th is there if I want it. I really feel like 4.10's would be ideal for me since i rarely cruise over 70mph. I personally don't like to cruise over 2600 rpm's I feel that my 4.0L's sweet spot is there.

 

I'm goinf to swap on my aluminum wheels, and get a good tire balance, and go for a drive to see one more time, but I'm pretty certain 4.10's are all I need with 31's.

[Eagle]

 

You don't take into account the large increase in tire mass?

 

In terms of revolutions per mile (which is the only important criterion when matching gears to tire size

 Sure, but that doesn't show up in a tire size chart. What I'm saying is that every tire/gear chart I have ever encountered (other than mine) bases their calculation on the diameter of the tire, without taking into account sidewall squat that makes the actual rolling radius considerably less than half the mathematical diameter.

 

Once you really know what gears will result in what RPMs at ___ MPH, then you can decide for yourself how much allowance you want to make for heaver tires.

 

I agree that 4.10s are a good choice with 31x10.50s. For reference, as I have posted numerous times, I have 3.73s in my 88 MJ 5-speed. The overall drive ratio is EXACTLY the same as my wife's stock XJ with automatic and stock tires. It's fine for daily driver duty, but 4.10s would be much better off-road, and perfectly acceptable on the street and highway. For anything larger than 31x10.50, I think 4.56 is pretty much the minimum unless money is truly an object and you get a deal on 4-cylinder axles already set up with the 4.10s.

 

But the OP here is asking about tires that are 31.6 inches in diameter. That's equivalent to a 32, not a 31. 31x10.50s typically have an actual outside diameter of about 30.5 ... and 30x9.50s typically run about 29.5, making them barely larger than a 235/75-15. It's important to be certain what your tire size really is before making decisions that are going to cost money and that you'll be living with for a long while.