Just remember that a gps is only accurat when driving on flat roads. When driving on hills or mountains it won't even be close to actual speed.

 

I have a garmin and just stock tires. My speedo and my Garmin usually stay about the same till i get around 60mph then the Garmin tends to read a couple mph faster than the speedo.

 

When I take the same Garmin unit and use it in my Sierra 1500 it shows 60 Mph on the GPS and only about 58 Mph on the truck's speedometer. No hills where I'm from. I guess there must be an acceptable range for automotive instrumentation.

 

My speedo used to read slower compared to my cluster in my blazer. Was pretty close on my 91 and dads 93 trucks. My cluster finally died so I just use the tomtom now and have never had any problems even with hills