causcg

Hey folks, I have 3.8l automatic X. Currently has 3.21. According to the chart, with my new 285s (roughly 33") I need 4.88s for best over all performance. That seems a bit low Huh? Doing my own calculations I need 3.73 , 4.10s would be perfect (nearest to stock performance) or 4.56 if I needed the extra power for crawling, which I don't. What am I missing. Throw me a bone.

Canvas

I have 5.13's in 2011 JKU because I am going up to 37's in a couple of months. I would look down the road to see what your final tire size is going to be. With that being said and your previous statement that you do not crawl I would look at 4.56's simple because it will give you the flexibility to upsize your tires and if the even you sell your Jeep you have a better selling point. Regear once so you do not have to pay twice.

R/
Will

causcg

Thanks will. 4.56 is what I was thinking, but why would the chart say 4.88. Are we both missing something?

casamtb

Keep in mind those charts are based on the actual mathematical heights and gear ratios. Not the tire height listing. Assuming you have 285/70r17 (32.8") the tire is actually going to be an actual 31.8-32.0 inches. That being said, 4.88 would be overkill.

causcg

Sweet. Thanks for the input. If I ever went 35s which would be years if I even did it, 4.56 ok for those as well?

shabbernigdo

yes. the older 3.8 needs more gearing due to the lack of power thats why they list 4.88. for 33's 4.56 should be fine. like the last guy said they almost never are the listed size. normally a little under it.

nthinuf

Consider all the variables, including the elevation where you live, how and where you drive, how much extra weight you carry or have bolted on, and your personal preferences for power/driveability. There is no perfect one-size-fits-all answer.

I ran 5.13's with 33's that measured 32", then 35's that measured just under 34", 5.38's with those 35's, and now I am on 37's that measure around 36". Screaming high rpm's at any time? Nope. Around 2650 @ 70 mph for the highest rpm combo. Which is close to what a bone stock 3.8 6spd rubi was running right off the dealers lot. High enough that this 3.8 auto is turning enough rpm to make enough HP and Torque to get my 4door moving down the road. Not nearly enough rpm to kill the mpg's or burn up the motor.

Many people may disagree, but it is hard to over-gear these 3.8 auto's...

Gafatboy

I've got an 08 unlimited X, 3.8 and auto. 35" toyos. Just want to gear enough to keep it in OD, with stock 3.73 it just won't stay in OD. If I remember correctly tire set at 33.75 to get speedo correct. I'm in South Georgia with only small hills. Would 4:56 hold OD or would I need to go deeper

Riflejk

I would go with 4.88. Two things spend your money once and you really can't over gear the 3.8. The transmission is geared high and gearing your axles is the best thing you can do to a 3.8.

Joe_B

Don't fear the gear.

I've run 33s with 4.10s and it was ok but I wouldn't spend the money to regear the jeep to 4.10s. Go with 4.88s and don't look back.