inaccurate speedometer

Seems like all cars run 3-5% optimistic on their speed readout compared to actual.

[lagrasta]

Here's the thing that really threw me off. I have a ScanGaugeII OBDII scanner that has a gauge mode that can, among other things, display the speed according to the car's computer.

I compared that, the speedo, and a gps unit and got this:
Speedo = 75 mph
Gps = 68 mph
OBDII = 69 mph

That confirms for me that the on board computer knows the correct speed and is intentionally under-reporting it on the speedo. At least, I know that the computer is reading the real road speed correctly.

I know manufacturers calibrate speedos conservatively, but I thought it was interesting to see the difference between the speedo and the OBDII readings.

[voltairesb]

I have a '97 box, and I can't seem to get the hvac into diagnostic mode. Is there a trick?

[RandallNeighbour]

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.