speedometer inacurate

I just took my 99 suburban in to have the speedometer calibrated. It is 3.5-4 mph slow, just enough to bother me. I bought the suburban used with p255/70r16 tires already on the rig. I took it into the dealer and they reprogramed the tire size from the original tire size to the p255/70r16 and sent me on my way. There was no change. I brought it back and they did some road tests and tried drastic tire sizes to see if there would be a difference. There was no change in the speedometer calibration regardless of what tire size was entered.

The service person attempted to explain that it might be a PCA or PCM or something and something about sensors. I kind of went blank and saw dollar signs coming out of his mouth. He said it was safe to drive because there were no check lights or codes.

Has anyone run into this before?
Or can anyone explain in layman terms what may be wrong here?
Is there a fix that is less than $500 or am I just going to have to recalibrate my brain?
Any insight would be appreciated.

My 1999 also has the same problem about 4 mph slow. I would appreciate any help in getting it back to the right setting. The tires are the correct size but not the original ones.

I took the suburban into another dealer and they found that the previous dealer had left and oversize tire and rim size in the computer(thanks). They reset to the correct size. They also found a bulliten on the problem. The bulliten said that the computer is receiving the correct speed and the error is in the speedo head. It uses a magnet to turn the dial and there is error that cannot be taken out. It wasn't clear if the speedo uses the same info as the computer to calculate speed. The manufacturer allows for a 6% error. That would be 3 mph at 50 mph. It appears that the only fix would be to send in the speedo head for service or replace the speedo head. I guess I will have to live with the error.