there not, cheap radar

 

best thing to do is. if you know a cop with laser. have him shot you at a set speed. their guns are supposed to be calibrated daily. I don't know a cop to ask.

 

the gps is pretty accurate. your speedo cannot be faulted for being a little off. think about it. when your puter is programmed for a particular tire size it determines the outer circumference of a tire size and reads it to you via your speedo. do you think a michelin and a cooper and a goodyear even though they are all the same size on the label are actually all the same circumference. i am willing to bet that all michelins of the same size and model aren't the same circumference.

then you have to consider wear and tear. a new tire will have a different measurement than one that has 40k miles on it. the truck is not going to vary its readings relative to your wear and tear. its only going to read rotations of 360 degrees. it doesn't know if you have a new tire, a used tire, a michelin, a goodyear, proper inflation, etc., etc. there are far too many variables on the speedo, therefore use the gps. i haven't gotten one ticket when the gps says i was going the speed limit. knock on wood.