Speedometer problem. Fix/Replace?

[avgjoeracing]

My speedo reads low. It is consistent but not linear. 40 MPH indicated is 52 actual, 50 ind = 67 act, 55 ind = 75 act. The needle doesn't bounce around much and it's been like this for years so I believe that the speedo cable is just fine. I have 15" wheel/tires, TH350 and 4.11 in the rearend (option listed on glove box door tag thingy, I need to get a pic of that). I don't have a tach. The actual speed was obtained via GPS. I know the trans was rebuilt in '78 (a time when they actually rebuilt stuf not remanned). I originally thought that the speedo gears were replaced with ones for a standard rearend and not the 4.10 but after much cervesa I've decided that the inidicated speed would be higher than actual not lower.

Am I in left field, is this a common problem, can it be fixed is it worth the $75 to get a new one,...? I'm letting my fingers do the talking before I crawl under and mess w/ my DD and only running vehicle. I did do a search and only found that someone gave up on fixing the speedo but not how.

Any help would be great,
Joe