99 4dr 200,000 miles blazer with normal oem tires and no lights, codes, or issues. The speedometer is 10mph too high at 60mph and 15+ too high at 80. I checked speed with my gps and an app on my phone..why?

Mike

 

A lot of speedometers read high. I always check mine if I get a new vehicle. (I haven't bought one in 20+ years but that's beside the point) Is the odometer reading more than it should? You can check that easy on the interstate. The mile markers are set out by surveyors and age EXACTLY one mile apart. Don't go by the sign that says a town is 15 miles away. Those get put where ever handy.

It's possible the gearing was changed at some time in the past or it came from the factory with taller tires and wheels. If you've put lower profile tires on the same diameter rims, that would cause that too. For instance, if you went from 75 series profile tires to 60 series, or maybe even 50 series, the overall diameter of the tire/wheel will be less and it will turn more to go the same distance.

 

You must have never been to AZ, the illegals put the mile markers in, neh I say any more.

 

This is something new.......I used an app on my phone and checked it on another car and a gps. Th blazer is waay off. No tire changes