With my over size tires it stay the same on mine, not 5mph off at 30mph then 10mph at 60mph ect its always the same.

 

chances are its not the truck, but the GPS. they are not as accurate as you think they are when it comes to showing how fast you are going. They are all the same, since they use satelites to determine your whereabouts, and your truck is actually touching the ground....don't be alarmed!

 

The police also have numerous speed indicators around the area to help with speeders, and they read the same 5 mph off---

 

and even their radar guns are off most of the time...though they will never admit that! easiest way to get out of a ticket is to ask when they and the gun were last certified

 

I thought my TomTom was wrong, my brand new '06 Sprinter van was off by 5mph, then it went in my '97 DI, it was off by 5mph.
Then I put it in my '89 Camry and it was dead on.
So although GPS is not 100% accurate, it is still close.

 

GPS variance is 0.1 - 0.5 MPH And I believe NAS speedo tolerances allow discrepancies in excess of 10% of aggregate reading. So long story short, GPS is far more accurate than most speedos.