Which of these statements is correct about recalibrating for larger tires with an automatic tranny?

[BillyHW]

Okay, so I've read the internet trying to get the right answer to this question, but there appears to be a few answers that people give in roughly equal frequencies with roughly equal passion, and I don't know who is right.

Which of these statements is correct about recalibrating for larger tires (or higher axle or transfer case ratio) with an automatic transmission?

1. Automatic transmission shift schedules do *not* need to be adjusted, because they are based on engine RPM, not vehicle speed. When you are recalibrating you just need to make the speedometer read correctly so that you don't accidentally get a speeding ticket because you didn't know the true speed you were going.

2. Automatic transmission shift schedules *do* need to be adjusted, because they are based on engine RPM *and* vehicle speed, but the computer will automatically adjust these schedules once you recalibrate the speedometer so it is getting the true correct vehicle speed.

3. Automatic transmission shift schedules *do* need to be adjusted, because they are based on engine RPM *and* vehicle speed, and you need to reprogram these shift schedules in the computer yourself with some sort of calibrator tool that can do this in addition to just recalibrating the speedometer and odometer readings.

I'm hoping the right answer is #1 or #2, because that would make life simple. But if it's #3, does anyone know if the Tazer, ProCal, or Mopar Calibration Service reprogram the shift schedules correctly, or do they just recalibrate the speedometer and odometer so they read correctly? Which tool does the shift schedule reprogramming too?

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[kkuntz01]

Simply put, if you’re putting larger tires on a vehicle you need to recalibrate the computer to account for them. There are several systems that are affected when adding larger tires. One is the ABS/ESC system. If you don’t calibrate, it doesn’t operate as it was designed and doesn’t operate the braking system in a safe or efficient manner, especially in an emergency situation. Just throwing out random numbers, your computer thinks you have a tire that is spinning at 605 revolutions per mile when in reality they’re spinning at 553 revolutions per mile. The other system that effected is the transmission computer. The way I understand it is there are two data points that impact automatic transmission shifts, engine RPM and vehicle speed. From the factory the computer is to up shift or down shift when engine RPM and vehicle speed intersect at various points for every gear. When you add larger tires your speedometer is now registering a lower speed than it should resulting in transmission not shifting when it should. For the people who only made a small jump in tire size they’re not likely to notice much in the way of issues. The guys who made a big jump in tire size, they’re more likely to notice issues.

 

[LLRubylady]

How about Manuals? Is it the same way?
Good point about ESC. I didn’t even think about that.
Dealer told me I had to have it done but I’m just curious as to what else gets fixed besides speedo.

 

[1quick1]

I can say when I went to 315/70/17's I could no longer get into 8th gear and my speedo was off about 6-7mph at hwy speeds. The dealer calibration fixed both of those issues so it seems it does something with the shift points and the speedo.

 

[kkuntz01]

How about Manuals? Is it the same way?
Good point about ESC. I didn’t even think about that.
Dealer told me I had to have it done but I’m just curious as to what else gets fixed besides speedo.

Most people think of re calibrating the computer for tire size as just something they do to get to the speedo to show the correct speed or get the transmission to shift properly. Those are big things that need to be addressed. The lesser thought about issues are how the vehicles safety systems will function, or rather not function if the computer doesn't compensate for larger diameter tires.

Then of course there are potential legal ramifications one can face if they cause a bad wreck and it's determined it could have been avoided had the vehicles safety systems operated properly.


The joys of modifying a vehicle, all the little things we have to think about. My buddy once compared modifying a Jeep to buying your wife a new dress. You can't just expect to buy her the dress and be done with it. There are all the things that she's going to need to go along with that dress that you see and don't see.

 

[LLRubylady]

Most people think of re calibrating the computer for tire size as just something they do to get to the speedo to show the correct speed or get the transmission to shift properly. Those are big things that need to be addressed. The lesser thought about issues are how the vehicles safety systems will function, or rather not function if the computer doesn't compensate for larger diameter tires.

Then of course there are potential legal ramifications one can face if they cause a bad wreck and it's determined it could have been avoided had the vehicles safety systems operated properly.


The joys of modifying a vehicle, all the little things we have to think about. My buddy once compared modifying a Jeep to buying your wife a new dress. You can't just expect to buy her the dress and be done with it. There are all the things that she's going to need to go along with that dress that you see and don't see.

Very true

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