Gear ratio

The speedo reading depends on a few things, so I recommend you determine the actual rear diff ratio by marking the driveshaft and tire and counting the relative rotations. Then based on your tire diameter you can determine the correct speedometer gear to get the speedo more accurate.

you have four things at play here ... one is the rear ratio, the second is the trans output shaft speedo drive gear, the third is the speedo cable driven gear and the fourth is tire size .... I would first see if the rear ratio can be determined by jacking up one side of the axle (jackstands and all for safety), put in neutral and rotate the lifted side tire a goodly number of times whilst counting the revs the driveshaft makes .... should do like thirty turns to ensure you get enough revs of the driveshat to work off any errors ..... then do the ratio of driveshaft to wheel turns .... guessing that it should be 2.7 something or perhaps 3.0 unless someone change the R&P gears or entire rear .... a 3.25 rear replacement of an OEM 3.0 would give you about what you're experiencing .... then try to compare that against the door plate that details the axle ratio ... if that matches up, then are you running a smaller tire than OEM ??? wouldn't think so as 20 MPH @ 60 is about 33% and I can't see a tire size being off that much ..... then it boils down to the tranny and speedo cable gears .... and you have to get at those to check for color code or count teeth .... hard to do for the tranny drive gear as it's buried inside the tailshaft .... Good Luck - hth ..... 67GTFB