My speedometer appears to read ~15% too low. For example, when the speedometer indicates 55 mph, I am actually going close to 65, and when it shows 80, I am flying past traffic doing about 95 mph. This error has been verified by pacing with friends' cars.
The vehicle in question is a 93.5 Supra TT with auto transmission. It rides on stock size Enkei RP01s with 235/45 front and 275/40 rear. I know going to different profile tires makes a difference, but it shouldn't be so dramatic.
How can I recalibrate the speedo?
-Rudy