OK, I put 285's on on 2001 and used Hypertech to tell the truck I had 32.85 inch tires. When I went through speed radar signs in PA and NJ, both told me I was traveling 61 MPH when the spedomoter said 70 MPH. Any idea what is wrong and any idea what size tire I should tell Hypertech/my truck I have to bring things back to right? Thanks.

 

Ok, assuming you have 285/75R16 tires (which is a 32.8 inch tire, so you are correct) then the spedo should be working right. I would have no idea why not, maybe a bad unit?? However, check your tires again for the exact size of the tire, and make sure it is correct. Use this site to check for the proper tire size - http://4lo.com/calc/gearmetric.htm

Good luck!
-Dan

 

Thanks. Your link has a section that allowed me to enter my current tire size, the speedometer speed and the actual speed. It is telling me to program the truck for 28.6 inch tires. We will see...

Thanks again.

 

if that don't work try getting a different gear for your speedo.My truck had 295/75/16's on it when i bought and i noticed there was a speedo gear in the glove box.don't know what would be involved in getting 1.