well the cruise on a van works at 35. the speed o meter is not correct on most vehicles so 35 is really 32-33. you need to also use the gps ground speed reading to determine the real forward velocity of your vehicle.
I used to be a median stalker, so I will tell you this! GPS is
USUALLY more accurate than a vehicle speedometer, but is not accepted in a court of law. Case law dictates that a GPS is not accurate enough to say it is correct. Use your head, use your speedometer, GPS, fixed object & stopwatch count, and one of the best things is to use one of those mobile "your speed is" trailers. Don't go by your GPS alone however. Courts will accept "my speedometer said" they will not accept "my GPS said". Food for thought.
My truck is 2.5 miles under what the speedometer shows, I've checked by multiple means.