Hey...
How do you know that your speedo is actually giving the right speed? I have a Garmin iQUE 3600 which is Palm device with integrated GPS, and I made some interesting discoveries while driving different cars:
My Acura TL-S is dead on regarding speed compared to the GPS
MY Volvo XC 70 is indicating 10kph more than actual speed, but it is progressive: under 50kph, the difference is less than 5 kph. That's with the original rubber and wheels.
I'll test this as soon as I get the my GT tomorrow.
One niffty little thing with the Garmin is that one of the guys on a Garmin forum developed an application that can calculate your 0-60, 1/8, 1/8 mile, and offer an autostart or the treelight configuration. The application's name is Time Trial. Tried it with my Acura and it's quite fun. I must test it against an actual run at the drag strip - will wait for the GT to do so.