Tire size help 2008 F250 - U7164

[jaskmartin]

I guess I'm just being dense but I can't seem to set my tire size. I went from the stock 275/65/20 to 35 x 12.5R20. Can you please walk me through the process step by step.

[mikel]

Enter 317/60/20

[jaskmartin]

I can't get to where to enter this. Please give me a step by step to get to that point. Sorry to be such a pain.

[mikel]

Sorry...

Select the tune, then go to modify tune, then select modify parameters, then select tire size.

Once you have entered the info, press esc until the tool asks if you'd like to install the tune now and press yes, all set.

[jaskmartin]

I found where I can input Revs per mile, I come up with about 578. I think this did it. Thanks for the help.

[eastwoodcb]

I just got the U7164 and it has no tire size selector in the parameters. It does have revs per mile. How do I change my tire size to correct the speedometer?

[mikel]

Revs per mile is determined by dividing 20618/tire height in inches.

[eastwoodcb]

I get 589 when using that formula. When I tried 575 it didn't change. I did 480 it still didn't change. I checked it by 2 gps units. Both were the exact same, speedometer 55 actual speed by both gps units is 60. What can I do?

[mikel]

Are you reinstalling the tune after making the changes?

[eastwoodcb]

Yes. Still doesn't work for me.