Reprogramming ECU for larger tires (35/37)

[Quick]

Many of us are upgrading to larger wheels/tires - I recently put 35s on my 2019 JLUR. I read somewhere that someone had it reprogrammed to improve the shift points in the transmission. Is this something the dealer offers? Does it really change the shift points of the transmission or is it only correcting the speedo?

I don’t mind the speedo being off (not great, but whatever), but if it improves the shifting of the transmission, that would be beneficial.

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[ormandj]

The dealer option does exist. It reprograms the tire size, which is used by the computer control systems for ABS, transmission, etc. You'll want to get it done. You can go aftermarket and get something like the Tazer JL and do it yourself, as well. I did it myself, and it's not that difficult. I suspect AEV/etc will have options that are fairly straight forward to work with, also.

 

[Uhdinator]

I used this: https://www.roughcountry.com/speedometer-calibrator-90019a.html

The cable that came with mine plugs into any open slot in a terminal block behind the glove box.
I reads codes first as it has to clear any codes before cal is done. (so it can be used to clear CEL too for minor things that might cause an erroneous code from time to time.)

I used my iPhone propped on dash with a speedo app. Checked at various speeds and got it within 1 mph.

Measure actual tire height and use that and not the mfr (unloaded) listed size.
Keep in mind the ABS, traction control, trans all work with calc of wheel speed and rpm etc...so you want to re cal so all these systems work well. The trans is also adaptive in that if you have a heavy foot, the trans adapts by gradually adjusting/averaging data to adjust shift points. If you drive like a granny or lots of city driving, the trans will lower the rpm shift points overtime.

Guys in the dodge challenger forum caught onto this and realized after doing lots of city driving/commuting that their car seemed more lifeless over time. The fix was to pull the fuse once a month, for the Trans control unit and re insert after a few minutes to reset the TCU. This made the car feel like it came to life and the adaptive learning starts over.

You might try a battery disconnect for a few minutes then do the re cal. If it won't stay in 8th gear at interstate speed, try driving with a lighter foot and it will adapt to shifting at a little lower rpm.

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