What are the pros/cons when changing gear ratios?
Right now i have a 2000 with 4.0 auto on 31's with a 8.25 rearend.
I believe that has a the 3.55 gears?
well when the lift and tires went on MPG's seemed to go down alot...
one of my buddys said i should regear it to maybe a 4.10 and i could get some better gas mileage, is this true?
If so what would be some other benefits/flaws of this?
Thanks!

 

There are no downsidies to re-gearing. Installing lower gears (numerically higher) will get your RPM's back down at highway speeds and get your engine back to the performance level it had with stock sized tires.

 

I agree always a good idea...

 

Actually, at cruising speed, the 4.10s will raise your rpms. Deeper gearing means higher rpms.

This does not mean that the 3.55s are the optimal gearing. As you probably already know, you are not keeping your engine in the power band, especially in stop and go traffic.
Its a give and take situation. The 4.10s will help you off the line (city traffic), but will raise your rpms on the highway.

If you sit down and do the math: new tire diameter/old tire diameter X original gear ratio

In your case: 30.8"/28.23"X3.55=3.87

You'll see that to regain the exact same gearing as stock you're looking at a gear ratio of 3.87. However, no one makes that size, so you would go to the next closest size. That just happens to be 4.10. Now that isn't too bad as it's only 6% higher then the optimal gearing. So, at cruising speed, you'll only be running your engine at 6% higher rpms. ie: (2120 rpms vs. 2000 rpms)

Ok to summarize.

Yes, technically there is a con:
Highway gas mileage (at most a 6% drop)

But, that is far out-weighed by the pros:
Deeper gearing for a lower crawl ratio
Better City gas mileage
More power

 

Did I mention I tend to over think things?

 

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

http://www.4x4offroads.com/gear-ratio-chart.html

 

Hmm i seem to think 3.55's with 31's on the xj is fine? I cruise at maybe 2100 rpms on the highway and have plenty of power.

 

I have plenty of power, but the MPG's seemed to just drop from about 20-21 to maybe 14 in the city, i might be over exaggerating but i clocked my last tank and got 60 on a quarter tank when i use to get about 100.
so i just wanna get some better MPG's i have the intake,
spark plugs and oil change comes tomorrow.
next is exhaust and checking 02's.
and then ill get back to you see if there is any change.
Thanks!

 

what tire pressure you running? that's a BIG drop in MPG and i wouldn't think it were the gears doing that.

 

Im running just over 55 PSI, and they are 265/75/R16 BFG All-terrian TA KO's. i know have the lift and tires are going to hurt the MPG but i didnt think this much. Im going to clock another 1/4 of tank to see if anything changes.

 

I was thinking maybe you were running low pressure but 55 is quite high. Does it ride rough at that psi?

 

nope, the tires say a max of 65 and my tire guy said to run 55-60 but 60 seemed to high so i left it at 55.

 

Not to go off topic but there's a misunderstanding with tire pressure and jeeps. IF you do the calculation, you'll see that you only need about 1/2 or so of the stated sidewall max psi. If you are driving a full size heavy truck loaded down, then you'll need somewhere near max psi. Running that high of pressure in your tires on a jeep will result in center tread wearing down faster than normal. Just a heads up

I've learned to never listen to the tire guys anywhere because they always inflated them to max psi which rides rough and wears down the tire faster. Probably a tactic to get them to come back for new tires sooner. I dunno but most of them don't even bother to calculate the correct psi for a specific vehicle.

Most people run 25-30 psi with 31" tires on Jeeps. I run 22 psi sometimes 25 (highway) and get perfectly even treadwear. I'm not trying to get you to change your psi, just letting you know that it's an awful lot of psi for that size.

 

I thought that was high, but my dad manages a car dealership/bodyship and his technician did the mounting/balancing aligning etc. (as it was his boss's car) but i think i will take them down to 40-45?
Thanks for heads up!

 

I run my 31x10.50X15 BFG A/T's at 32 psi. They do good for me.