Speedometer Inaccuracy???

[53helofe]

I have a Garmin NUVi 765 that has its own spee display and it is always less than the speedometer. Below 40 mph the vehicle indicates about 2 mph less and up to about 100 mph it indicates about 3 mph less and at 120 indicated on the vehicle the gps indicated 115 mph.

I honestly believe the gps is accurate.

I do not know what wheel size the vehicle originally came with but since my spare is 1 inch smaller than the other wheels perhaps I should suspect that is my cuplrit.

I am not near the vehicle now so I can not post what is on the vehicle and what is in the "trunk".

Anyone else running an aftermarket gps that disagrees with the vehicle speedomoeter?