For some reason, IDK Y my OE speed o started reading the incorrect speed. Tires were OE at the time. After I got my flash tuner, I set the speed o to the correct MPH with my phone and GPS. When I did the Denali cluster swap, ( Denali have a 120 MPH speed o) again the speed o was off, so I again had to reset the speed o with my tuner. Thing is, I have been having my truck for 18 years. I know that when I bought the truck when it was 5 years old, in 2002, the speed o was correct. Still have the same rear end with the same gears.