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 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.
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