speedometer vs gps

[m8silverado]

My Predator says I am going 63 MPH yet my GPS says 67 MPH. I tend to think that the GPS is right but are they totally accurate? That is quite a variance to me. Any ideas would be appreciated. I have a 03 Silverado 1500 with 4.10's and 285-75-18 Toyo Mt's. They say they are 35" tall but are more like 34" or a gnat's behind shorter.

[Alfanooos]

try to properly adjust the tier size on predator

[mikel]

do you have the gear ratio set or is that stock?

[m8silverado]

The 4.10 ratio is the stock one. When I put the tire size off the sidewall in, then it is really way off. I actually did rolling circumference and started from there to get where I am now. If I remember right I am at a 33" tire to have speed match my GPS. Now I'm wondering why so much difference and here we are.

[neonhomer]

Neonhomer - back from the dead... LOL

I found my stock calibration, on stock tires and stock gears, was a couple of MPH off compared to my GPS. (TomTom Go 320s). I also compared it to my older Magellan Sport Trak and both my GPS's read the same.

My theory - The factory calibration is purposely set slow.