I put 265 on my truck and had the dealer tell my computer what size tires I have. When he was done I turned my GPS on and my speedo is still off. Did he do something wrong?

As far as I know, dealers are not allowed to reprogram the speedometer.

I've tried two dealers & they both told me the same thing. Some people claim that they've had theirs reprogrammed by the dealer, but I haven't seen a dealer reprogrammed truck in person....

I have a Predator that I use to reprogram the speedo.Edited by: conradv

How much is it off by?

I stood there while he hooked up the tool to my computer so he did do something. It off by about 2 miles an hour. Nothing to big, but it annoys mehttp://www.dieselplace.com/forum/smileys/Geek.gif

That'd be great if he did - at least he tried.

I have 255 tires, which are bigger than the 265's so I think my speedometer would be off by 8% or so.

With the predator, it's exactly what my gps tells me.

I hope someone comes out with a tuner for the lly soon, I want to play too!!!http://www.dieselplace.com/forum/smileys/Cry.gif

For some unknown reason, GM dealer's equipment only adjust the ABS brakes for a different tire size - it does not affect the speedo adjustment. I've read many posts where folks say that after-market reprogrammers are the only current way to ajust the speedo for different tire sizes. I've also read that some of those reprogrammers, although they advertise to make ajustments for tire size, don't seem to actually fix the problem.


One way to look at it -- bigger tires make your odometer reading a little low, extending your warranty.


You know, perhaps that is the reason that GM doesn't let their dealers reprogram the speedo setting. If the dealer sets his buddies odometers to read less miles than traveled, they get a little more warrenty.

I put 265s on min e and bought them from the dealer and they reprogramed it and everything is correct.

I have not switched to 265's yet, but I think my speedo actually reads fast with the 245's on it. When I do 70 in my car, very few pass me, but when I do 70 in the truck, lots pass by. I had the same issue with a 1989 4x4 with 245's. Went to 265 and seemed to be right on. Also, my 1995 Z-71 with 265's was also right on. Never tested any of them with a GPS, though.


Maybe GM has all the speedos programmed between the 245 and 265 so they don't have to change it for the two sizes? When I put my 265's on, I think my speedo will be right on.

I hope someone comes out with a tuner for the lly soon, I want to play too!!!http://www.dieselplace.com/forum/smileys/Cry.gif


Me too!http://www.dieselplace.com/forum/smileys/Cool.gif

i have 315's no interest in correcting. off by about 12.6%, just do a quick calc to estimate speed. extends my warranty an additional 3K+ miles.....http://www.dieselplace.com/forum/smileys/Clap.gif