I recently swapped the C6 from my '81 F250 with the C6 from my '88 F250. (the '81 tranny was recently completely rebuilt) The '81 has a 3.0 rearend and the '88 has a 4.1. Now the '81 shows 47mph at 60mph and the '88 shows 77mph at 60mph. Since I used the speedometer driven gears that were on the cables (17t on the '81 and 19t on the '88) I assumed everything was going to be fine. Apparently the output shaft on the tranny can have 6, 7, 8, or 9 teeth and the output shaft is chosen for the rearend at initial vehicle assembly time. I guess these two trannies have driving gear counts of 7 and 9 (7/9 is the ratio of the speed errors). I was able to get a 21t (I need a 24t) driven gear for the '88 which now shows 64mph at 60 mph. I need a 13 tooth gear for the '81 but according to the local Ford dealer, they only make them from 16 to 21 teeth. An option is to swap the output shafts but that means dissassembling the transmissions completely because the tailshaft comes out forward through the case so that's not really and option. I've heard about a device to calibrate speedometers that is essentially a tiny gearbox that mounts in line with the speedometer cable. Does anyone have a clue where these devices are available and how much they cost? Thanks in advance.

 

Sounds like you have it figured out.
I remember seeing what you are looking for when I built my '54. Try looking thru the Classic magazines, or at MId-fiftys Ford or Sacremento Vintage Ford. That's were I got most of my parts for my '54.

Winford