Just my 2 cents...

if the speedometer is off, it wont be 'by xx mph' but will read a percentage error- correcting by moving the needle might 'fix' it at a CERTAIN MPH, but it will still be off by a percentage everywhere else- just the percentage will be plus or minus the new 'calibration' point.

I'd suggest the post mentioned way up- use 60 mph, and the milemarkers/stopwatch to count them off, if youre not comfy with that, do 10 miles/10 minutes or such to minimize start/stop errors...the highway markers are usually darn close. if you can adjust speed by percentage of error, and verify by another highway run, then just find someone with a tuner to increase/decrease tire circumference(not diameter) or rearend ratio parameters by same percentage, and it should be right on. If you have little to no tire wear, and pressures are OK, you could even easier verify by tachometer(if its correct that is...)

 

I guess you are referring to the other posters who's speedo seems correct but on eval is not actually. In my case, the problem was just my needle did not even sit at zero but instead above it. Adjusting the needle took care of my issue perfectly.