I doubt the wheels are the issue. I measured the circumference and height of my 17's and my 18's and they're very, very close. Not enough to make 5 mph difference.
BTW, on my 97 with 18's on it, my GPS, digital gauge, and needles are all the same. Go figure.
Palo Alto Speedo in central California will calibrate speedos for a couple of hundred bucks including shipping if anyone is genuinely annoyed with this problem. I actually wish mine were calibrated too high because it helps reduce the likelihood of receiving a hand-written invitation to visit with a judge in person, and with checkbook in hand.
