Regear Or Bigger Tires?

[onlyinajeep726]

Alright, I have a bit of an odd question.

 

Some background.  My truck started out with a 2.5L and AX4.  Gear ratio from factory is 3.54.  It's now a 4.0L and AX15.  I'm still running the same axles as was equipped when the truck was built.  I know that Jeep used 3.07's for a 4.0/5-speed combo for optimal fuel efficiency.  I'm currently averaging about 14.8 MPG mixed driving.  I started a new job which is 30 minutes away and most of the trip there is on an interstate going 65-70 MPH.  It revs at ~3,000 RPM.

 

The question is, would it be better to swap to 3.07's or run slightly larger tires to bring the revs back down?

 

Before anyone mentions tuning it up and cleaning stuff, I've essentially done all of that.  New air filter, new brass cap and rotor, new wires, plugs, Neon 703 fuel injectors, new O₂ sensor, new battery, new brass battery terminals, ground refreshments, etc.

[Spectormj]

I'd regear

Bigger tires will suck down the mpg to around 12

[AeroNautical]

4.0, ax15, 3.55 and 3000 rpm at 70? Something doesn't seem right. I run that exact set up, with 29x9.5 tires and it revs about 2300 at that speed. Could it be possible, when you swapped the 2.5 for a 4.0, that you forgot to calibrate your tach?

[onlyinajeep726]

I'm sure my tach is off.  I just kind of eye balled it based on idle when I calibrated it back for the 4.0L when I first started it up back in October... :doh:

 

But that doesn't change the fact that I get just shy of 15 MPG consistently when it should (in theory) getting about 18-19 MPG.  Gear ratio is the only thing it could be, at least that's my guess. :crossfingers:

[benjy_26]

Ummmm.... something aint quite right. I'm putting down an average of 15.8 in mixed city and highway driving in my ZJ with a 4.7 stroker,al auto tranny, 3.73's and 32" KM2's. How's your A/F mix? Not running too rich?

[onlyinajeep726]

Yea, I know somethings not right.  That's why I'm here lol.  I don't know WTF is going on with the gas mileage.  A/F ratio should be fine.  Doesn't run lean or rich, the best I can tell.

[Pete M]

it's not all that cut and dry.  when I replaced the blown 3.07 d35 in my 88 (2wd/4.0/stick) with a 3.55 geared d44 my mpgs got better.  don't believe the hype. :D  there are plenty of reasons why your fuel economy might be a bit lower than it should, from off sensors to off calculations.

[Incommando]

To echo Pete: Low numerical gears do not automatically equal peak MPG. Having a balanced package that matches engine hp/TQ level to final drive ratio often increases mileage depending one your usage.  

 

If you are calculating MPG based on the odometer could it be inaccurate based on your swaps?

[onlyinajeep726]

The speedometer was compared against my GPS for accuracy and at 55 MPH, it's just a hair off.  I imagine at 70 MPH, it's might be showing 69 MPH on the speedometer.

 

With my current size tires and gear ratios, I should have a 36 tooth speedo gear.  I think my current one is 35, but that shouldn't be enough to give me a huge discrepancy in miles driven.

 

Also, the reason I was thinking that 3.07's would help because that's what it would be from factory.  They would've optimized it based on final drive ratio for best blend between fuel and torque.

 

By the way, I'm using Fuelly.com to calculate my MPG.  Here's my profile: http://www.fuelly.com/driver/onlyinjeep726

[Pete M]

the factory is NOT to be assumed to be the perfect anything.  they gave us the dana 35, the peugeot, the CAD, the god awful wiring for the headlights, the zip nuts that break off inside the frame, the plethora of leak spots to get water under the carpet, the bumper hitch made from sheetmetal, and the hideous-to-drive 3.07 ratio in the axles. 

 

:thumbsup:

[yxmj]

I personally think you would be ok with changing your tires. It would be easier and cheaper than a gear swap.

 

 

The change would not be as dramatic. Just some KISS math.....225/70/R15 = a perfect world tire diameter of 27 3/8".....If you upped them to 235/75/R15 (PW diameter of 28 7/8”) an increase of 5.2%

 

 

While changing the gearing from 3.54 to 3.07 would be an increase of 13.3%.

 

 

 

IMHO it would be way cheaper and easier to find a set of 235/75/R15’s (maybe a buddy has a set sitting around?) and try a 2 or 3 day trial run to see if it helps.

 

 

Again the math will not work perfect but it should drop your RPM to from 3000 to around 2800 ( which IMHO sounds high.....should be around 2500 :dunno: ).

 

 

Just My 2 cents  :thumbsup:

 

 

[JeepcoMJ]

How about a torquer cam and new tires. Guaranteed mpg improvement if you can keep rpm down. Mustang 5.0 injectors, open up tb, etc. etc.

[Tactical Bacon]

Something sounds off to me. Mine runs about 15 mpg before adjusting for oversized tires. Comes up to like 18 mpg. According to the sticker in the glove box, stock size on mine was about 26 and now I'm running 31s. Stock 3.07 rear gears behind a 4.0/AX15 2wd combo. Ridiculously undergeared. You should be doing better than that. Also, I agree on upsizing tires. Easier, looks better, and cheaper.

[nitroxsteve]

I have an 88 4.0 5speed with 3.55 ratio and 235/75R15 tires and get 19mpg and some time a little better. I would not regear to try picking up a couple mpg. If you need new tires and wanted to go up a size I would probably do that but I wouldn't throw away any good tires to try it.

[92tanMJ]

The numbers next to the gear are the speeds at X rpm

 

With 235/75/15 (28.8's) with 3.55: (what I'm assuming you have now)

Gear:  1500 RPM/ 2500 RPM/ 3500 RPM
1  9  16  22
2  16  26  36
3  25  42  59
4  36  61  85
5  46  77  107

 

With 275/75/15 (31.2's) with 3.55: (what I'm assuming your going to get)

Gear  1500 RPM 2500 RPM 3500 RPM
1  10  17  24
2  17  28  39
3  27  45  64
4  39  65  92
5  50  83  116

 

 

With 235/75/15 (28.8's) with 3.07: (what I'm assuming you have now)

Gear:  1500 RPM/ 2500 RPM/ 3500 RPM
1  11  18  26
2  18  30  42
3  29  49  68
4  42  70  98
5  53  89  124

 

 

With 275/75/15 (31.2's) with 3.07: (what I'm assuming your going to get)

Gear  1500 RPM 2500 RPM 3500 RPM
1  12  20  28
2  19  32  45
3  32  53  74
4  45  76  106
5  58  96  134

 

 

IMO I would NOT regear down.. if anything regear up and get bigger tires :thumbsup:

 

*All of the rpms/speed are right for the 31's. My XJ has 3.55 and MJ has 3.07*

[a4xnut]

My '89 has 4.0L (HO head, 99 intake & Pacesetter header) with AX-15 and 3.07's and 235/75R15's.  I get ~16mpg (corrected for speedo error - which is 8% off) when running around empty.  5th gear is useless unless I'm on the freeway over 60. My plan is to regear AND run larger tires, but I'm going the other way with the gears. 

(My old XJ is supplying front lift, 4.56's & lockers) :thumbsup:

[92tanMJ]

FYI I get all around better mpg in my XJ with 3.55 then my MJ 3.07

[onlyinajeep726]

On my XJ, I have 31's and 4.10s.  On the highway, I average about 14.5 mpg.  On the MJ, with 27's and 3.55s, I'm averaging just above that on highway... The XJ is an automatic and the MJ is a straight drive... :dunno:

[92tanMJ]

Have you one adjusted your speedo biased upon the larger tires? and two then adjusted for youre smaller gearing?

[onlyinajeep726]

Both of my Jeeps have the correct speedo gear based on tire size and gear ratio.  GPS verified at a range of speeds to ensure accuracy.

[glundblad]

This won't help much but it takes a lot of horsepower to turn bigger tires.  On the other hand, lugging an engine doesn't get you too much either.  I have a 4.0 and 3.07.  I treat it like a 4 speed.  Sometimes, I can get it into 5th but if I hit a hill, it won't maintain it.  If you were to change gearing, going to 3.07 would be the last thing I would do.

[92tanMJ]

4.0/ax15/3.07/31's  I use 1-5 everyday and I'm not even on the highway. It has no problems going into 5th at ~60 for me :dunno:

 

Both of my Jeeps have the correct speedo gear based on tire size and gear ratio.  GPS verified at a range of speeds to ensure accuracy.

 

 

Figured you had it covered but just wanted to check :thumbsup:

[onlyinajeep726]

I think I figured out part of the problem by solving another problem.  The engine has always idled higher than normal after it has been warmed up, since I swapped it in.  Figured I had a vacuum leak, so I went to searching.  Then it occurred to me, check the manifold bolts.  Sure enough, they were loose and so, I tightened them, and my idle has went down noticeably.  With that said, I've scored more miles on half a tank of gas than previously, so I think my average MPG has improved, but I won't know for sure till I run a few tanks down.

[Eagle]

Some background.  My truck started out with a 2.5L and AX4.  Gear ratio from factory is 3.54.  It's now a 4.0L and AX15.  I'm still running the same axles as was equipped when the truck was built.  I know that Jeep used 3.07's for a 4.0/5-speed combo for optimal fuel efficiency.  I'm currently averaging about 14.8 MPG mixed driving.  I started a new job which is 30 minutes away and most of the trip there is on an interstate going 65-70 MPH.  It revs at ~3,000 RPM.

Something doesn't seem right.

 

According to the BF Goodrich web site, a 225/70-15 tire runs 758 revolutions per mile. That's very close to a typical 215/75-15, which runs 754 revolutions per mile. With 3.54 gears and an AX-15 transmission, at 70 MPH you should be turning 3114 RPM in 4th gear, but only 2335 in 5th gear.

 

Did you recalibrate your tachometer when you swapped from a 4-cylinder to a 6-cylinder engine? I don't think your tach is giving you accurate readings. I think you are actually turning closer to 2000 RPM.

 

The factory didn't choose 3.07 gears with the 5-speeds for optimum fuel efficiency in the real world, they chose those gears to get better results on the arbitrary and artificial government economy test. In real life, 3.07 gears suck and 3.54 or even 3.73 gears should almost always result in better fuel economy on stock size tires. 3.07 gears have the engine running well below the torque peak at highway speed, and that's NOT the way to obtain better economy.

[onlyinajeep726]

 

Some background.  My truck started out with a 2.5L and AX4.  Gear ratio from factory is 3.54.  It's now a 4.0L and AX15.  I'm still running the same axles as was equipped when the truck was built.  I know that Jeep used 3.07's for a 4.0/5-speed combo for optimal fuel efficiency.  I'm currently averaging about 14.8 MPG mixed driving.  I started a new job which is 30 minutes away and most of the trip there is on an interstate going 65-70 MPH.  It revs at ~3,000 RPM.

Something doesn't seem right.

 

According to the BF Goodrich web site, a 225/70-15 tire runs 758 revolutions per mile. That's very close to a typical 215/75-15, which runs 754 revolutions per mile. With 3.54 gears and an AX-15 transmission, at 70 MPH you should be turning 3114 RPM in 4th gear, but only 2335 in 5th gear.

 

Did you recalibrate your tachometer when you swapped from a 4-cylinder to a 6-cylinder engine? I don't think your tach is giving you accurate readings. I think you are actually turning closer to 2000 RPM.

 

The factory didn't choose 3.07 gears with the 5-speeds for optimum fuel efficiency in the real world, they chose those gears to get better results on the arbitrary and artificial government economy test. In real life, 3.07 gears suck and 3.54 or even 3.73 gears should almost always result in better fuel economy on stock size tires. 3.07 gears have the engine running well below the torque peak at highway speed, and that's NOT the way to obtain better economy.

 

 

The tach is off and needs to be calibrated.  It started out in the XJ that I used as a donor, then I calibrated it to the 4-banger, then I just guessed when I redid it for the 4.0.  I need to redo it the correct way.  But, like I said in my post just above yours, I think I may have figured it out but I will know for sure in time.