trying to get the 72 comet roadworthy,and was unable to find a thread that suggested as to how to get the correct gear for steeper than stock? My combo is t-5 and 8.8 3.73 trac-lock from a 95 explorer. The original gear from the 3 speed manual trans (was 3.00) looks 15 to 20 mph off. Knowing comets didn't come any steeper would a mustang from the same vintage be the fix?
using the info on this page http://www.fordracingparts.com/download/charts/129.pdf you should get a 21 tooth gear.
You left out one important piece of the puzzle here: the rear tire diameter. Tire diameter is half of the rearend's final drive ratio.
3 more teeth will give you about 10 mph. 21 tooth is the most Ford offered then you had to go with a reducer. Taller tires will make the speedometer read lower. I remember reading where the speedometer could be as much as 5 or 7% off from the factory...until they went to electronics.
I gave up on trying to get the speedometer correct, and just put my old TomTom One GPS in there. I have the 21 tooth gear, and still 10% off.
Using that formula Bryant posted a link to, you need a 20 tooth gear. Your revs per mile works out to be 758 with 295/60/15 tires.
