Tire size, speedo and Automatic

[alrola]

hi

I have a mopar 2” lift with 35” ko2 x 12.5 x 20. I had the dealer set the speedo as part of install.
When I drove it home I noticed the thing would not go into 8th gear. I did some research and the 35 Ko2 are 34.5 so when I got my Tazer MINI I looked and the computer was at 34. I set it to 34.55. I also went down on psi to 32. Car rides nice but seems to stay on 7th gear even more. If I look at the gas pedal wrong it downshifts like I’m racing.
How do those things above affect the auto transmission? I have looked at the charts on the forum and I seem to be well within acceptable with 410 gearing on a Rubicon.
Will going to 35 help on programming?

Thanks

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[Uhdinator]

1. The advertised tire height is unloaded/unmounted mfr spec. You have to measure actual height as mounted and under load at desired PSI. Likely why the dealer set it at 34. I ended up with a setting at 1/4" lower than measured height to get iPhone speedo app to be within 1 mph of speedo.

2. The transmission program has adaptive learning. If you drive with a heavy foot, the trans gradually changes shift points to higher RPM. This means you are defeating yourself as you have trained the TCM to not shift until you reach a higher rpm. If you drive with a light foot consistently it will gradually change shift points to lower rpm.

3. Measure your tires. Pull fuse to TCM for a few minutes and re install fuse to reset adaptive learning. Enter measured tire size. Drive easy to nearest interstate on ramp. Accelerate as gently as possible to 70mph and set cruise, check speedo with I phone speedo or gps app.

4. If it goes into eighth gear but sea-saws........move the shifter to manual mode when in 8th and it won't down shift.

5. if it seems to shift to 8th better than before.....it's likely a heavy foot and possibly tire size that's defeating it from using 8th gear.

 

[Carlton]

You need to measure the tires under the weight of the jeep. Your 35s probably measure out to be about 33. Use the measured height for the Tazer.

The only way to keep the same gear ratios as stock is to regear the differentials. It will not have the same performance otherwise.

 

[WranglerMan]

You have to understand those new tires are a lot of rolling mass so going to larger tires that are taller, wider and heavier than stock are going to affect your speedo which can be corrected with a programmer or dealer flash, the transmission with relearn shift points but as far as 8th gear and only seeing every so often is a reality unless you regear which is not worth the expense unless you go to like 37’s

I started my stock Sahara getting 8th gear all the time and once I got my lift, tires and front bumper and winch 8th gear is not seen very often and I have dropped about 3-4 mpg but hey it’s not a Prius

 

[JC7]

How much does your combo weigh?

You could try airing up a little too.

 

[alrola]

Thank you all.
If it hangs in 7th will not harm anything?
Sorry I came from an AUDI RS5....no clue on this jeep stuff. Jeep is a lot more fun though !
So the speedo calibration will not affect how the transmission sees the car and shit ie 70 when it’s gong 65 or vice versa ?

 

[WranglerMan]

Thank you all.
If it hangs in 7th will not harm anything?
Sorry I came from an AUDI RS5....no clue on this jeep stuff. Jeep is a lot more fun though !
So the speedo calibration will not affect how the transmission sees the car and shit ie 70 when it’s gong 65 or vice versa ?

Speedo calibration does affect how fast it shows you traveling so larger tires equate to rolling faster than it shows and your odometer will be off, you best best is to have the speedo calibrated for correct speed, for me when I put 315’s on from the 255 stock tires I was rolling 5+ miles faster than the speedo indicated plus with larger tires you don’t want to run with the 35-37 lbs of pressure as you will be riding on the crown, you mileage will be better doing as you have less contact with the road so it’s best to do a chalk test

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