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

[Andy4639]

Not a hard fix. You need to get the right tail shaft gear and speedo gear in the transmission. Any good transmission shop can do this and if they know the rear gear numbers and tire size it's a breeze. Very common problem on these trucks with all the differant tires and tran swaps.