'92 Custom w/ '95 MAF 5.0 M/T, 33's, 4.10 LSD
Mine is the opposite. I have the PSOM calibrated to my tire size, which has it reading accurate at highway speeds (anything north of ~45 mph), but inaccurate at around-town speeds. 30 mph indicated is actually 25 mph, 35 mph is a little over 30, 40 mph is probably 37 or 38, and then it evens out from there. I've never done a GPS test; that's all based on speed camera displays. GPS can be wildly inconsistent from day to day, in my experience.I went out and did a test with a GPS, apparently 35 actual is 45 on my speedo, so it's not as bad as what I first guessed from the eyeball test, but the difference does get worse at higher speeds. Though from what I read that's normal?
If I could get the giant brick hurling faster than 80 mph, and had the speedo range to capture it, I'm guessing it would start reading inaccurate in the other direction.