Re-gearing the AX15, effect on MPG?

[plinkerneil]

So I've got a busted axle, and I'm thinking about taking the opportunity to upgrade. I've got a '94 4.0L, AX15, stock 3.07 gears, on 225/75r15 tires. I'm thinking about re-gearing at 3.55 and putting a Detroit Truetrac in the rear diff.

But here's the thing--I currently get 23 to 24 MPG on the highway when driving at anywhere between 65 and 80 mph. (Speedo is very close to correct, I verified it with GPS.) With my current setup, that puts the RPMs in a range from 1850 to 2200 in 5th gear. If I go to 3.55, the same speeds will be at roughly 2200 to 2700 RPM.

I was looking through GoJeep's website, and found this page, which says that theoretically the ideal efficiency 4.0L should be found between 2110 and 2640 RPM. If this is true, the 3.55's should either improve or have no effect on gas mileage, as long as I'm willing to drop to 78 mph instead of a full 80 mph. But I'd be interested in hearing from somebody with real data.

Does anybody have any experience with how this gear change might change the gas mileage?

 

[chpmnsws6]

It'll lower mpg. They put the 3.07 gears in the highway trucks for a reason.

 

[DanMan2k06]

3.07 to 3.55? You might lose like 5 miles per TANK of gas. There are so many variables in calculating mpg's there's no way you would even notice.

 

[plinkerneil]

Thanks for the opinion, DanMan. May I ask what you're basing that ~5 mi/20 gal figure on? I would agree that 0.25mpg is within the margin of error.

 

[DanMan2k06]

I'm basing it off of the final drive ratio and speed vs rpm of a AX15 with 3.55, vs AW4 with 3.55. With the AW4, at 2500rpm's you're spec'd at 79mph in OD. With the AX15 you're at 75mph in 5th gear (both with 3.55's and NP231 case). Two different transmission's yes, with the AX15 in favor of mileage. But having driven/owned both I can tell you ultimately it's nothing to make a fuss about. Sure, there is a difference. But it's so close to your original setup that I think you won't notice at all. Upon first installation you'll have a placebo affect and think you can feel a big difference, but in reality it's very little. My buddy did this exact swap thinking it would give his 33's a little more pep until he got gears. He still hated it, and ended up going with 4.56's.

And chp if you're referring to big rigs, that's a TOTALLY different ballgame. Of course a diesel with a turbo the size of a beach ball isn't going to need steep gearing...

 

[plinkerneil]

True, I did those calculations too. The AX15 5th gear is very close to the AW4 4th gear, so by switching the AX15 over to 3.55 I would have essentially the same final ratio as a stock XJ with auto. Unfortunately, your friend's experience doesn't help me. I'm sure it didn't work with his 33's, since that re-gear wasn't enough to get his engine up into the power band any quicker. But I'm on stock tires. Anyway, it's not really about the extra torque, I just would really like a geared LSD in the rear and they're not made for the Dana35 at 3.07.

Theoretically there should be little change in the highway gas mileage (maybe a little improvement around town), but I'm hoping somebody has done this on stock tires and can fill me in.

 

[DanMan2k06]

He had the stock tires on before the 33's, still very little (if any noticeable) difference. I remember a thread about this exact topic about a year ago, I'll see if I can dig it up.

 

[plinkerneil]

Good to know his experience on the stock tires. That'd be great if you can find the thread. I've been searching the last two evenings and only seem to turn up discussions on what ratio is best for crawling or racing. Which is interesting, but...

 

[VAhasnoWAVES]

your numbers sound a little off to me, throw in the numbers for the specific gear ratios:
http://www.csgnetwork.com/multirpmcalc.html
http://www.novak-adapt.com/knowledge/ax15.htm

your looking at about a 200rpm difference jumping from 3.07s to 3.55s. if the price is right, i say go for it. but i wouldnt regear to 3.55s. what i did was swap in a set of 3.55s from an automatic. im happier with 3.55s on 31s than i am with 3.07s on 31s. if you find that 3.55s is too much for 225/75s, then get a set of 235/75s or 30x9.50s for your next set of tires. both should fit stock fine. but i think youll be happy with the tad lower ratio.

is it something to marvel over. no... worth regearing? i wouldnt say so... especially when all the axles are out there already. then your looking at regearing the front as well (if its 4x4).

your speedo will be way off too.

 

[plinkerneil]

...worth regearing? i wouldnt say so...

Heh. I don't have a choice about getting a new front axle, courtesy of a crazy-driving lady in a KJ that is probably on its way to a JY now. It's easy to find 3.55 D30's from an automatic, not so much 3.07's. The question is whether I'm going to re-gear the front or the rear. If I re-gear the rear to 3.55, I'm probably going to put in a Detroit Truetrac for a little help in bad weather.

Ah, here's a thread with good info. "5-90" points out that the peak torque/power on the 4.0L comes in at about 2800. That torque peak should also be the efficiency peak, and he claims good results after quitting the use of 5th gear, and that 3.55 would have been a better ratio for the AX15. There are other posts in that thread from people who saw gains in MPG by going to consistently higher RPMs on the highway.

That all sounds like I'd be safe going to the higher ratio. Anybody else want to chime in?

 

[VAhasnoWAVES]

spending money on a truetrac, gears, rebuild kit... and then installing... is going to cost a lot more than just swapping in a 3.55 geared rear end.

your priorities and funds may be different than mine though.

 

[plinkerneil]

True dat. My priorities and plans should probably be a different thread, though.

 

[gradon]

I ran auto 3.55s on 27" tires with my ax15 and it is how it should've came from the factory. I have a stroker so mileage won't be comparable(lucky if I get 250mi/tank).

 

[plinkerneil]

I ran auto 3.55s on 27" tires with my ax15 and it is how it should've came from the factory.

Yeah, I've been reading about your rig. :worship: No mine isn't going to be comparable to yours, and it's possible our driving styles might be a bit different. Not to mention, your stroker torque peak must be at lower RPM than stock. But it's good to hear your opinion on the gear ratio.

 

[kastein]

I'd do it. Hell, you can get 3.55 geared axles for like 200 bucks all-in here on the forums. Pick up a set (get a 29 spline 8.25 rear, avoid the 27 spline and d35, no reason to buy a weaker axle than you can get for the same money) and install them, if you don't like it, go back. Swapping a pair of axles is basically a one day or weekend job.

If you get the 8.25 and a geared LSD for it, you can run it open to see if you like the gear ratio, then swap the LSD in after. Since you are only swapping a carrier at that point you will only have to set backlash and carrier preload, which is done with side bearing adjusters instead of shims on an 8.25. Also the 8.25 doesn't have a carrier break anywhere near the ratios put in jeeps (it's down around 2.something iirc) so as long as the LSD you get covers 3.07 it'll cover any other gear ratio you could ever put in that axle for a jeep.

3.55s, an AX15, and 33s on my MJ is definitely undergeared, btw. 4.10s would probably feel almost stock but if I'm gonna spend money on gears, I am going 4.56 or 4.88.

It'll hit 103 still (fastest I've wanted to go lifted with driveshaft vibes) but it takes its sweet time getting there.

 

[XJEEPER]

plinkerneil, is your rig a 2WD?

 

[plinkerneil]

No, it's a basically stock D30/D35. T/C is a 231, but I'm going to change that. The D30 is bent and chances are that the replacements I find will be geared 3.55, which is why I'm thinking about this. I'm also going to keep my ABS brakes, so I'll be sticking with the D35 rear unless somebody tells me a really easy way to swap to a bigger axle while keeping the stock tone rings.

 

[Breezio69]

Find an xj with 3.55 at the junk yard and buy both axles... around here a "cheap" ring and pinion swap is $250, (Teraflex, which is local here, charges $400) thats without the cost of the ring and pinion set itself... at junk yards around me a complete front axle is about $185 and a rear is I think $20 less.

And like someone mentioned before, if you found an xj in the junk yard thats a 97' up, you'd be getting a better front as far as U joints go, and a better rear end if it was an 8.25... for less than the cost of ONLY the ring and pinions.

and not EVERY part you get from the junk yard needs a rebuild. I found a front drive shaft for my SYE in a rear ended xj, and the driveshaft had paint marker on it stating a shop and RO#, I called them and it was rebuilt by them about two weeks before the xj was sent to the scrapper (according to the paint marker on the fender of the xj), $34 new and balanced driveline for me. guess it was my lucky day

 

[Breezio69]

As far as keeping the ABS.... WHY?

 

[plinkerneil]

No off road, it's strictly a daily driver. That's why this is in the "Street and Performance" forum. I've managed to steer through a couple of pretty scary situations in bad weather, partly thanks to the ABS.