It's tough to say. I run 3.73 gears on 35" tires, but I also have a 5 speed manual. But a basic rule of thumb is to get the best gas mileage to keep the tachometer under 2k. As long as your cruising and not doing a whole lot of accelerating, OD should be fine. Of course you are going to need to step on the gas going up hills because you can't downshift an auto trans, but I wouldn't say that is a bad thing. Unless your trip is mostly uphill.

 

I ran the same set up for a few years....42rle, 33's, and stock 3.73 gears.

I did not turn the OD off while running around town below 40mph. If I knew I was going on the hiway, or running on a road over 40mph, I hit the OD off button. It's not good for the tranny or your gas mileage to have it lugging like that. The OD is basically useless with that tire and gear ratio. I never used it.

 

I'm running 35's with 5.13 gears with the 4.0 and 42rle. I turn my OD off when city driving. I turn it back on at about 55-60mph on the freeway. I may be wrong but the engine lugging cannot be good.

 

Your engine still "lugs" in OD with 5.13s??

 

I have the 3.73 with 4rle and 33's and over drive seems useless. Do you find the 4.88's too low? I am trying to decide between 4.56's and 4.88's. Wonder if the 4.88's would be too low and effect mileage some.

 

3.73s w/ 33s is useless. you loose overdrive entirely.

4.88s is what you want for 33s.

 

The shifting back and forth out of overdrive builds heat. Not good on automatics. Turn it off.

 

Thanks for the advice everyone. I figured it wasnt good on the engine to be switching overdrive on and off to get up to speed. I'm filling up a fresh tank today so I'm gonna keep overdrive off and see how it goes.

 

Well i apologize for the misinformation. I didn't know there was such a big difference b/w manual and auto trans when it comes to gear ratios. I just figured OD on = 5th gear for me and OD off = 4th gear for me:doh:, and coming from personal experience, I figured that if I do fine with the same gears and larger tires, then you should be fine.

 

I am buying a 2004 Unlimited with an auto. trans., 4.88 gears and 33's. Think I should use overdrive on two lane roads going 50-55 mph?

 

That is the ideal gear ratio for your tranny, and tires. You should be able to just hop in it and go. Let the tranny do it's thing. If it bogs down on a steep grade or something, turn the OD off until you get on flat ground again.