I just bought a 2010 Silverado 1500 4x4. It had bigger tires on it that the previous owner put on it. The speedometer is of by 20 mph, but when the speedometer reads 100 I’m actually going 70. I have used a speedometer calibrator and i have also taken it to two separate mechanics who cannot figure it out either.

 

My first thought would be that there might be a non-stock set of gears in the diffs. Otherwise bigger tires should actually affect the speedometer in the other direction.

You can get a rough idea of your gear ratio if you jack up the rear axle, mark the driveshaft and count the rotations it takes to get one full revolution of the tire.

 

go into the ebcm with a scan tool and confirm the tire size set for the vehicle