So I recently put new tires and rims on my truck. 275/65r20. My speedometer is messed up as expected. My tuner has the ability to change the tire size but I haven't gotten it to work for me. Big O tires told me I have 34.75 inch tires, Tirerack.com had a chart saying I have 34.5 inch. When measured by hand I get 33inch. My stock size was 32.5 and with the new tires my speedo was off 4mph. I have my chip set at 34.75 and my speedo is off 2 mph from the GPS on my phone. Any suggestions to get my speedo correct again? Thanks!

 

If you're only off 2 I wouldn't worry about it.

 

Not too worried about it since it's already been about a month but I just wanted to see if anyone had the secret formula to solve the problem. Such as if your speedo is faster make your tire size in the tuner larger. I'll play around and see what I can do.

 

My speedometer is off 2 mph from the GPS in my van and its got the factory tires on it.


Somebody told me theres a 2 mph variation in a GPS on purpose but I don't know if that's true.

 

I think thats only a 34.1 inch tire

275(0.65)=178.75mm
178.75mm/25.4=7.03 in
7.03in(2)+20in=34.1 in

 

Thanks guys. I'll have to set my chip to a 34 inch tire and go from there. I also never tried my phone GPS with my stock tires so I'm kinda shooting in the dark by testing it with my new tires. I'll keep you posted.