The dealership told me that putting larger tires on my Renny will eventually jack up the tranny. Unlike other jeep models the ECM isn't able to reprogram with different tire sizes. The speedometer and odometer are based on the revs per mile of the stock tires which are 28" on a Trailhawk. They claim a larger tire will throw the timing of the shifting and eventually causes the tranny to malfunction. But here's the numbers. The revs per mile of a stock tire are 721. That is 1 rev is about 7.3 ft. For a 29.5" tire there are 683 revs per mile. 1 rev is 7.7 ft. So on the larger tire you are actually travelling about 293 ft per mile more because the ECM measures 721 revs per mile. Roughly your odometer will be off by the length of a football field over a mile. Over a 100,000 miles your odometer will roughly be off by 5,500 miles less than actually travelled. When driving at 70 mph on the speedometer you will actually be going about 74 mph. The question is does it really have an effect? In theory I suppose you will lose torque and power but is it noticable? I'm running 225/65/17 KO2s which have a 28.5" diameter in part because I fell for the argument that it would make a difference. Now I'm not so sure.
I call BS on what your dealer is saying. The ECU can only read rpm of the wheels versus rpm of the motor, along with throttle position and etc, and set shift points accordingly. It has no way of knowing that you are actually traveling 74 mph vs 70 mph due to tire diameter, as far as I know.
Yeah I don't know if the ECM really cares one way or another what size tires one runs and I don't see how the transmission would suffer from it. It wouldn't make a difference in sport mode. I shift under normal acceleration at about 2500 rpm regardless of which gear I'm in and at about 3000 rpm under aggressive circumstances. In automatic mode it shifts at about 2000 rpms under normal acceleration and at about 2500 rpm when being aggressive. For what it's worth the dealership told me installing a lift would mess up my suspension and that putting on spacers would prematurely wear out the ball joints. Neither of which has come true yet.
