I have read just about every post on gearing but I am still a little hesitant so I'm going to ask.

I have a 2010, 2Dr, Auto, 3.5" Lift, with 3.21 gears. We run 35x15 BFG AT's in the summer and stock 255 75 17's in the winter. This is a daily driver with about 15% highway driving or so but for that 15% it's like driving a tortoise down the highway with the 35's! To be clear this is my wife's DD and it's really a Mall Crawler and never off road. It's just the look she wanted.

My question is am I going to love the gearing in the summer with the bigger tires only to hate it in the winter with the stock tires? Is the 4.88 going to be too low with the stock tires? I guess i should add I tow a two place sled trailer some weekends in the winter so it would seem the lower 4.88 gearing would help some there too.

Eric

 

As bad as these transmissions are I'd go deeper gears, but that's just me

 

4.88 gears with stock 32" tires would be almost not drivable on the street. Your RPM's would be through the roof. Your best bet would be 4.10. That will work with both tire sizes effectively. Night and day from the 3.21's.

 

Agree.... 4.10's

 

Another vote for 4.10, I drive 4.88 now with 35's and its a little too deep for me at 250Ft above sea level and mostly flat terrain. 32's would be a nightmare. I think 4.56 is the sweet spot for 35's, hoping 4.88 is the sweet spot for 37's!

 

Am I just misinterpreting the gearing charts for the 3.8l with an automatic? It would appear based on the charts that 4.88's would be in the "Good Range" for both tire sizes.

 

I run the 456 gears with truetracs front and rear. with 35 tires on my 14 sahara automatic. I have run with stock tires 255 70 18 with theses gears. not bad on the highway. My last jeep was a 09 with a 6 speed. also 456 gears. and 35s. when I traded it in had to drive 45 miles on interstate to the dealer stock 17 inch tires and wheels. it was great on the road. rpm wasn't too high. plenty of power. but I also live in florida. flat roads.

 

No, you are reading the chart right. You just need to read into the replies. Does the person have a newer JK with the more powerful 3.6 engine and a transmission with better ratios? Or a 3.8 with a 6-spd, which generally needs one ratio less than an auto. Or does he live in an area at sea level with flat roads, and just doesn't need the added hp/torque? Or whatever.

With a 3.8 auto, you will be ok with stock tires and 4.88's. (For my 4door 3.8 auto, I went to 5.13's with 33's, then moved to 35's, then regeared again to 5.38's. I live at altitude, and had a feeling that i would be going up to 37's at some point, which I did... But no issues with 5.13 and 33, or 5.38 and 35)


Just looking at the numbers (which does not take the extra drag from the lift into account, or the added weight/mass of the tires, or ...) you can do a fair comparison.

(using 31" actual measured height for the 32's, and 34" for the 35's)

3.21 with 32's
= 1681 rpm @ 70 mph

Press the OD Off button (OD is .69 so just divide the od/on rpm by that number)
1681 / .69 = 2436 rpm @ 70

4.88, 31" = 2556
4.88, 34" = 2330

So if I did my addition correctly, pushing od off now with the stock tires will put you within about 120 rpm of where you will be with 4.88's and od On. Close enough for a good comparison of how it will feel.

 

I should have clarified when I said that 4.88 was a little too deep. I have the 3.8L Manual. Sorry

 

Forgot about the OD you guys have on your 3.8's