Optimal gear ratio

[JMO413]

So the question of the day is what differential ratio would you run with 31's?

 

I have come to the point that I really need to regear my differentials. 3.07 are worthless with 31's. 5th grear might work on Nebraska interstate.Looking for opinions on what ratio you run or would run. The truck is usually my DD unless it's torn apart again. It sees the highway daily. It is my only truck so it does get to haul everything. Mud/field/snow/level B road is about the extent of of road. 3.55 would put me back to almost stock. But should I go lower? 3.73,4.11? It's allot of work to do twice. I know this is an opinion so all are appreciated.

[jeeptec1]

There is a formula for figuring out gear ratio for tire size. The original tire size on your Comanche was probably 28 inch. You now have 31’s. Your gear ratio is 3.07. 31 divided by 28 times you’re gear ratio of 3.07 equals 3.39. I reality that’s still not enough. The old rule of thumb used to be 31’s =4.10
33= 4.56. 35=4.88 etc. my experience says 4.10’s. I have an 88 Comanche with an automatic and 3.55 stocks gears. I have 31’s on it. It is ok. A freeway flyer. If you know your going to go wheeling and there is a possibility of going to 33’s in the future do the 4.10’s.


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[JMO413]

33 are a no go on this one maybe the next one lol. Not worth it on a DD. I found a similar equation on here.

OTD- original tire diameter

NTD- new tire diameter

CGR- current great ratio

EGR- effective gear ratio

OTD÷NTD ×CGR =EGR

28÷30.8 ×3.07 =2.79

 

[jeeptec1]

Thank 30.8 dived by 28 times 3.07 that equals 3.39


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[Jeep Driver]

4.10 for the win. 

[JMO413]

2 hours ago, jeeptec1 said:

Thank 30.8 dived by 28 times 3.07 that equals 3.39

I think that's backwards. 

44 ÷ 28 × 3.07 = 4.82

Wait 3.39 would return it to stock. Gotcha now.

[Pete M]

another vote for 4.10s   

 

let's say that 3.55s brings it back to "factory", but factory sucked so I want at lease 3.73, but then I want a bit more to compensate for the larger mass of tire and so 4.10s. 

 

also, I've driven an MJ with 4.10s and 31s and it felt perfect to me.   

[DesertRat1991]

I'm going to 32s and 4.10. Overgearing a bit to compensate for the camper shell.

[Eagle]

My personal rule of thumb is 3:54 for stock tires, 3.73s for 30" tires, 4.10s for 31s and maybe 32s, and 4.56 or 4.88 for anything larger than 32".

 

[Eagle]

37 minutes ago, DesertRat1991 said:

I'm going to 32s and 4.10. Overgearing a bit to compensate for the camper shell.

 

4.10s with 32" tires is hardly "overgearing." Stock tires with 3.54 gears turn 2336 RPM at 70 MPH. 32" tires with 4.10 gears turn 2375 RPM at 70 MPH. You're basically matching stock final drive ratio.

[DesertRat1991]

3 hours ago, Eagle said:

 

4.10s with 32" tires is hardly "overgearing." Stock tires with 3.54 gears turn 2336 RPM at 70 MPH. 32" tires with 4.10 gears turn 2375 RPM at 70 MPH. You're basically matching stock final drive ratio.

 

Interesting. When I ran the numbers a while back, assuming 26.5" stock tires (195/75R15) and 3.07s, I got 3.55 as the closest match to stock for 31's and ~3.73 for 32's.

 

[Minuit]

I'm not in your same situation, but I intend to regear to at least 3.73, more likely 4.10 for use with 29" tires. To match my original ratio I'd need 3.87s which don't exist.

 

When I do this, it'll be interesting to see if mileage changes, and if so in what direction. The HO 4.0 likes to rev a little bit, and I'm turning about 2200 rpm at highway speed currently.

[Eagle]

1 hour ago, DesertRat1991 said:

 

Interesting. When I ran the numbers a while back, assuming 26.5" stock tires (195/75R15) and 3.07s, I got 3.55 as the closest match to stock for 31's and ~3.73 for 32's.

 

 

I use numbers from my own spreadsheet, which I compiled using actual tire manufacturers' revolutions-per-mile data. I think that's more accurate than numbers based on a theoretical tire diameter. My spreadsheet is up on Google Docs -- there's a link to it somewhere in this site, lemme see if I can find it ...

 

See if this works: https://docs.google.com/spreadsheets/d/1pHFuhGgj6dQDfzyfFJH5z7NCDLW2KX3ABQgAJr3lBvM/edit#gid=0

 

 

FWIW -- I ended up running 3.73 gears in my '88 MJ with 31" tires. The gears were purchased for installation in my '88 XJ, which I ran on 30x9.50-15 tires. Then I bought the MJ, which had a 4" lift on it, so it was crying out for at least 31" tires. It was a 5-speed, though, and 31" tires with 3.07 gears and a 5-speed is rather awful. 4.10 gears would have been better but I already had the new 3.73 gears sitting there in a box, so they went into the MJ. The result was that the overall final drive ratio was almost exactly the same as stock tires with 3.54 gears. Very nice on the street, but I would have preferred the extra gearing of 4.10s off-road.

[WahooSteeler]

I have 3.55s, auto, 4.0L, 31s. As another poster noted, on flat highway it can stretch it's legs pretty nice. Off the line it doesn't jump but it's not bad at all IMO. My plan is to go to 3.73s and try to get a little better mileage than 4.10s but pull grades better and when I do some occasional hauling. The past week I have been using a friends dump trailer to haul a lot of brush to a dump site about 5 miles away. The 3.55s didn't seem t0o struggle to bad, I'm guessing the heaviest weight I pulled was 3500lbs between trailer and load. I don't do any wheeling, some mild trail running once in a blue moon so 4.10s don't seem necessary based on other threads I've researched here. Very interested in this thread.........

[Pete M]

engine rpm (and by extension gear ratio) is not the be-all, end-all to mpgs.    the actual math is far more complicated and it could turn out that 4.10s might give better fuel economy than 3.73s.  The rule of thumb is keeping the engine in the optimum spot of its torque curve. 

[Eagle]

5 hours ago, Pete M said:

engine rpm (and by extension gear ratio) is not the be-all, end-all to mpgs.    the actual math is far more complicated and it could turn out that 4.10s might give better fuel economy than 3.73s.  The rule of thumb is keeping the engine in the optimum spot of its torque curve. 

 

Correct. Which is why for stock tires 3.54 gears are a much better choice than 3.07. With 3.07 gears either you're below the optimum "fat" part of the torque curve at highway speed, or you don't use fifth gear. Seriously -- on stock tires with 3.07 gears, 65 MPH is 1800 RPM. Really? What were the engineers smoking when they made that decision? And that's with an overdrive ratio of 0.75:1 -- I think the ratio for the Peugeot tranny is 0.72:1, which just makes it worse.

[benjy_26]

How does this apply to 2.5L trucks? 

 

If I'm using 31's, should I swap in 4.56's? My stock rear end ratio is 3.55, btw. 

[Pete M]

2.5s always need a bit more gear than 4.0s    But I would also add in an AX-5 swap so you have overdrive. 

[Limeyjeeper]

This is a great chart that I have used. Does most of the legwork for you.

 

http://www.grimmjeeper.com/gears.html

 

[1989commanche]

I went to 3:73 on my Comanche with 31’s. Seems just about right for me. 

[gogmorgo]

I've got 3.55's and 31's in my 4.0/ax15 ZJ. It was definitely a noticeable change when I put on the 31's in place of the factory size but it's totally fine for a daily driver, IMO. I'm grabbing 4th going up hills more often, but I wouldn't call that necessarily a bad thing. I can also make it all the way across intersections before needing to shift out of first now. 

If you're towing with it though, I'd definitely go for something deeper. I was thinking that already on stock tires, haven't towed on the 31s yet, but I can't imagine doing any better with them.

 

I don't know what axles you're running, but if budget is a consideration you might want to look into where the carrier breaks are. Going from 3.07 to 3.55 crosses the carrier break in a d35. D30 carrier break is 3.73+, d44 is 3.92+. 

[JMO413]

2 minutes ago, gogmorgo said:

I've got 3.55's and 31's in my 4.0/ax15 ZJ. It was definitely a noticeable change when I put on the 31's in place of the factory size but it's totally fine for a daily driver, IMO. I'm grabbing 4th going up hills more often, but I wouldn't call that necessarily a bad thing. I can also make it all the way across intersections before needing to shift out of first now. 

If you're towing with it though, I'd definitely go for something deeper. I was thinking that already on stock tires, haven't towed on the 31s yet, but I can't imagine doing any better with them.

 

I don't know what axles you're running, but if budget is a consideration you might want to look into where the carrier breaks are. Going from 3.07 to 3.55 crosses the carrier break in a d35. D30 carrier break is 3.73+, d44 is 3.92+. 

I don't plan on towing with it ever. I have a WK for those jobs. Just stuff in the bed. I have a D35 and a D30. That leads me to the next step. Regear or swap out. Either way I only want to do it once.

[Jeep Driver]

20 minutes ago, JMO413 said:

I don't plan on towing with it ever. I have a WK for those jobs. Just stuff in the bed. I have a D35 and a D30. That leads me to the next step. Regear or swap out. Either way I only want to do it once.

I suggest you find a good pair of axles from a late model XJ.........3.55 are just fine. 

 

If  you wait, are diligent, you may find a pair out of a 2.5 XJ..........4.10s.........good all around gear for you.....and me.

 

You can obtain the best bang for your buck.........under or around $400 with new brakes and fluids, rear discs if you want to go all out. 

 

Good axles, good gears.  Hold out for LS rear, when it works it works nicely for moderate driving. 

[gogmorgo]

3.55 is easily the most common ration in XJs, any 4.0/auto will have it, for the d30. '97-'99 axles have bigger ujoints, but 2000+ is the TJ/ZJ low-pinion axle, which is less desirable. It also shouldn't be too difficult to find a 3.55 geared MJ d35, which wouldn't be horrible. That would probably be the cheapest and easiest route, more so than regearing existing axles for sure. But if you're not opposed to welding perches on, its not much more money or work to get an 8.25 out of a 97+ XJ (probably same one as the front) and you'll have a much beefier axle. 

If you can track down an XJ d30 with 3.73 or 4.10 gears, a KJ 8.25 can be had in either ratio and has disc brakes. 

 

I wouldn't bother regearing. You're looking at over $100 per axle for new gears, and another $50 for the bearings because there's no sense installing new gears without changing bearings, and those are the cheap parts, not the ones you'd want to replace stuff with. Then unless you're comfortable setting up your own gears, it'll be another couple hundred to pay a shop. Basically there's no point spending the money to regear unless you're doing a full rebuild on the axle at the same time... but if you can bolt in an axle already set up with the ratio you want that will probably be fine to run as is and bolt it right it?

¯\_(ツ)_/¯

[WahooSteeler]

15 hours ago, Pete M said:

engine rpm (and by extension gear ratio) is not the be-all, end-all to mpgs.    the actual math is far more complicated and it could turn out that 4.10s might give better fuel economy than 3.73s.  The rule of thumb is keeping the engine in the optimum spot of its torque curve. 

 

Absolutely. Which brings the question up of where is that on the 4.0L Renix and HO? For those that use their trucks as a DD and/or never "wheel" it, seems that would be the optimal starting point to shoot for. Certainly there are other factors of does the driver do mostly city driving? Highway? Reasonable mix? But again, seems like a logical place to start from and then if the driver wheels a lot, hauls a lot, tows a lot, etc then they adjust from there.