My father is using an edge insight cts2 I think it is. He is running 295/65/17 tires basically a 33in tire. He uses the insight for his speedometer as you can set it for oversize tires. He said the difference between what the truck speedometer shows and the insight is 2-3 mph

 

If you would like to fix the speedometer, use Rough country's speedometer calibrator. That is what I am using now to adjust my speedometer.