With the OEM tires, my car speedometer was always reading about 10 mph more at 70 than my GPS readout.

Now with 20000 on the OEM tires, I have changed the front two to Continental DWS's of the same 215 50 17 size. The car speedometer is now close to the GPS reading !!

Does this mean that the original Hankook tires were not even the correct diameter ???

 

Impossible. For just the tire to create a 10-mph difference it would' have to be over 3.5" shorter in diameter.

 

Thank you for your reply.

I do stand corrected - I should have said about 10 percent. I do remember when the GPS said 70, the speedometer would read >76.

215 55 17 tires are supposed to have a circumference of 82.659 inches.

A ten percent difference in diameter would be 2.631 inches

All I can say is that the GPS mph readout now matches the analog speedometer. There are two OEM tires yet on the back; I could try to measure the distance of one revolution and report back..