I've put on a set of 285/70R17 and used my flashcal to change the tire size but my question was about the stock settings. It was set to 30.25" stock size. It came with 255/75R17 which from what I've found is about a 32.1" tire. How would this have affected my vehicles behaviour? such as fuel economy rating and speedometer among others.

 

I would have expected that tire to be at least 31" tall. As long as you have it set right now it won't matter.

 

Pretty sure mine did the same thing. I just assumed it was defaulting to the smallest tire size for my year/model as I didn't think it was actually reading the computer to see where it was currently configured. Could be totally wrong but I know that 30.25 didn't make any sense. Speedo would have been wrong straight from the factory if so.

 

I just thought it was sloppy work from Chrysler but that isn't surprising. I've got it set now. According to a speedometer app I'm within about 3km/h I don't know if I can get it any closer

 

If you're really trying to get closer, you can use the following formula to get pretty darn close.

Circumference = Pi x Diameter.

1. Make sure your wheel is properly inflated to the PSI you plan to run.

2. Use a piece of chalk to mark the tread of your tire.

3. Roll the Jeep enough that the chalk makes two marks on the driveway.

4. Measure the distance between the center of the chalk marks. This is your circumference.

5. Divide by pi (3.1416).

This should give you the diameter of the tire for your programmer.

 

That's some good info. Thanks.

 

What people seem to miss is you can't just measure tire height. Almost all tires that are properly inflated have the load of the jeep on them. So you could measure hub centerline down to the ground x2. Thats why you will always see a smaller value in your programer to match GPS than just using tire height.

 

I will try that