I'm starting to get quotes for my next major upgrade to the Jeep. It has mainly to do with putting a custom Ford 8.8 axle versus a C8.25. A lot is unfortunately coming into play, like selectable lockers (more choices available with the 8.8). And, of course, while they're in there, they might as well re-gear the damn thing, right?

I'm a 3" lift with 31" Duratracs and have no real intention of going up to 33" in the future. I'm currently with a 3.55 axle and auto tranny. I was thinking about going up to 4.10, with it being the best all around ratio for around-town/highway/off-road driving. However, I'm quickly being talked into going to 4.56 for more power and better choices for upgrading tires later. It sounds intriguing having power going up hills & mountains! It's just the thought of doing 3000+ RPM to do 70mph scares the hell outta me!

I'm pulling 13 MPG now. What changes can I expect if I go to 4.10 vs. 4.56? It's more or less my daily driver and I'd like to do some extended highway driving to get to all the cool wheeling spots (2 hour drive minimum). Thanks for your input.

Your numbers are off by a little bit.

with 31's and 3.55 gears you should be turning about 2050 rpm at 70

with 31's and 4.10 gears you should be turning about 2350 rpm at 70

with 31's and 4.56 gears you should be turning about 2650 rpm at 70

Remember that the auto trans has an overdrive of about .75

I would think that adding the 4.10 will not affect the mileage , because the motor wont have to work as hard and will stay in over drive longer ...

IMHO ,going to the 4.56 only makes sense if you think you will be adding 33's in the future

use this link to calculate the numbers, remember that your OD (if automatic) is .75 not .70 like the calculator uses

If all you want is 4.10's then go to the JY & get a set of 4.10 geared XJ axles. I bet they would be cheaper than paying someone to mod a 8.8 to go into your XJ, plus a 8.8 is way over kill for 31's.

BTW, a 8.25 CAN handle 33's, locked on stock shafts.

31's & 13 MPG? Man, you need a tune up. I have 32's & 3.55's & get 16 MPG & ran 33's & got 14.5-15 MPG.

3k @ 70 is no big deal.
Personally I don't give a hoot ( <- pun intended) about MPG. I fill up my ride, drive it until it's down to 1/3 of a tank, fill that sucker right back up then rinse - repeat.

I just swaped an 8.8 under my rig with 4.10s I just did 800 miles and used less fuel on the trip than the last time I made the same trip with 3.55s and 30 tall rubber.

Everything in this post.

4.56:1 on 31s for highway? lol You'll need to install a Brownie box just to keep sane.

Thanks. I was going off a chart similar to this one:

http://www.offroaders.com/tech/gear-ratio-chart.htm

But I see how it can be misleading. As I said, I'm not inclined to go up to 33s in the future. But then I get to thinking, "what if?"

Tonight, I paid attention to my rpms. At 70 mph, I was at 1900 rpm. That speed is not adjusted for 31" tires. I think taking it out of OD brings it up to around 2300 prm.

I get jealous of all the non-California Jeepers. They all say that new axles can be had for under $200. Here in San Diego, I was lucky to find a 27-spline for around $300...lucky in that it had a lunchbox locker already in it. But you're still right. Even at $300 each, it's still cheaper than rebuilding both.

Oh, I've tuned it! Got the high quality cap & wires, Champion plus, etc. But I also have a 62mm TB, 99+ intake manifold, headers, 703 injectors, Magnaflow muffler, Duratracs, roof rack...all on 31's on a 3.55 axle. That's all gotta factor in somewhere, right?

But that 13mpg is based on a speedo that hasn't been adjusted for the bigger tires. Someone said to multiply the mileage by 1.1 to get a better calculation. I did that and found my new calculated MPG is 15. So maybe I just need to change the speedo gear...