I have a 15 GT Premium manual, back when I got the car it had 9000 miles on it. My rims are the stock 18's with 235 50 18's all around. My lug nut's are 21mm, but my speedometer says I'm going 2 mph faster than what I really am. First noticed this when I used the cruise control the first time. Any one else seen this?

 

My 2020 seems to be the same, GPS and speedo are about 2 MPH off...actually going slower than what the speedo says.