Hi All. Well, this problem has been plaguing me for some time now. But now that im restoring her i figured i would fix this. I purchased a rebuilt C4 transmission from a 1966 mustang. Never through too much of it but now i might as well fix it. The Speedometer gear is about 1.16% off (Behind). I think the reason for this is because the mustang had 14" rims and i now have 15" rims. Anybody know which gears to buy to correct the offset? I believe the tire size is 215/75R15 but i could be wrong.. I just thought I'd ask. Thanks for any help in advance!
If your speedometer reads too low, use a speedometer drive gear with fewer teeth. If your speedometer reads high, you need a drive gear with more teeth. Do this in one-tooth increments. A drive gear with 18 teeth is going to turn faster (and read faster) than a drive gear with 21 teeth. If your speedometer is reading 10 mph too high and you have an 18-tooth drive gear, you need a 20-tooth drive gear. The highest number of teeth available on a Ford speedometer drive gear is 21. After that, you must run a reducer that was common to Fords with 3.89:1, 3.91:1, 4.11:1, and 4.30:1 rearend gears. Taller tires will make the speedometer read lower. Shorter tires will make it read higher. Even tire inflation can affect speedometer reading.
Okay. Thanks! At 70 mph my Speedometer reads 60, so im not sure what tooth number i have but ill go in increments one lower.
what is the reducer can you post a link to it........ I am having the same issueI just installed 389 gears my speedometer is off about 15 miles per hour at 65 miles per hour but when I had 325 gears it would read correctly T.5 trans with 389 rear gears and 245/40/17's
Agreed.... A lot of variables here, may need to go three teeth... A speedo shop guy I talked with claimed each tooth was closer to 3mph, but really depends on tooth count of gear on output shaft and rear ratio...
I tried several and finally gave up and stuck my TomTom One in there permanently... I ended up one size shy of the largest (smallest?), which I was told would wear out quickly because of the very aggressive tooth pattern. I am still a good 10% off. Dead-nutz on with GPS!
