I have stock 93 RX7 and on my GPS it saying that I'm going 80 but on the speedometer it saying is going 85. I tried this on a different car and on the other car it matches with the GPS.

 

wheel/tire size?

 

I have OEM wheels with OEM sized tires, and my speedometer reads about 76 when I am going 70 MPH per GPS. Annoying but I just mentally adjust. Because the speedometer is electronic I assume the sender is just not accurate anymore after 20 years, it's on the longer and less important list of stuff to fix.

 

i have the stock wheels and tires

 

do you if there is a sensor on the transmission or is it the dashboard cluster that is not working. The car only has 30 thousand miles