Gearing question

chefdab · **Posted:** Sun Aug 06, 2006 11:11 pm

I always read on the Jeep boards about changing gears for larger tires to get back closer to stock ratios. My question is what if you ran say 4.56 gears with a smallish tire like a 31"? Wouldn't that effectively lower your crawl ratio in low range and give you better MPG? How would that affect regular highway perfomance? I'm thinking (hypothetically) that if you did something like this you wouldn't have to shell out the big bucks for a 4 to 1 conversion for the T-case. And save a little at the pump. Anybody know why this wouldn't work? If it would, how come no one is advocating something like this with gas prices being what they are?

uvalax236 · **Posted:** Mon Aug 07, 2006 2:18 am

4.56's with 31" tyres will give you horrible gas milage. The engine will be working harder than it needs to spin thoes small tyres.

chefdab · **Posted:** Tue Aug 08, 2006 6:07 pm

I just don't get it. Why would the engine work harder? I thought with gearing like that ( it's torque multiplication right?), it would have to produce less effort to turn the tire. So your RPMs would stay lower, ergo the mileage would go up. Kinda like when you downshift on a bike to go uphill.

05kjrenny · **Posted:** Tue Aug 08, 2006 8:28 pm

stock 31s on rubicon and 4.11 or 4.10 ? dont know what it is but 1st gear is useless on road, mpg? whats that
check out the rubicon owners forum and search for gearing theres some usefull info over there.

silver_2000_tj · **Joined:** Sat Jan 07, 2006 12:17 pm **Posts:** 29

Maybe this will help you understand a little better

I have 4.56 gears in my TJ pushing 33" tires, The RPMs are roughly 2500 at 65MPH and I get about 14 - 15 MPG, If I went 31" tires the RPM's would go up to get the same 65 MPH(somewhere around 3200) at which point your gas milage would go down because the engine is reving so high. On the other hand if I went to 35" tires the RPMs would do down ( 2200 rpms, gas milage would also go down because of the added weight/mass to move the larger tire. It's all relative to the gear size and the size of tire, the best advice I can give is go to a rmp/gear size calc like this and play with it to get a better idea.

http://www.ring-pinion.com/content/calculators/RPM.asp

btolle · **Posted:** Sat Aug 12, 2006 6:14 pm

Are you sure you want a 4:1 transfer case ratio? I have an 05 Rubicon and for most trails I would much prefer the 2.72:1 I had in the 04 Sahara.

True rock crawling in Moab is the only place I feel the 4:1 outperforms the 2.72:1. With anything less than a 45 degree angle the 2.72:1 would be my preference.

In Colorado I find myself using 4HI more often than I used to because 4LO is much higher RPM's than I want to turn in most situations including using the engine for braking compression when going downhill.

Now if I could just justify an Atlas 3 speed transfer case I could have the best of both worlds!

Gearing question

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