Speedometer gear?

[jes72mustang]

Well, Since I bought my 72 the speed-o has been WAY WAY off. It was reading about 80 MPH or more when doing 45MPH ( I was pacing my wife in the van). There were a number of mods done by previous owners, 4:10 Gears and wrong size tires (about 2 inches smaller in diameter). ANy way I put 225/70 R 14 on all around, and I replaced the 4:10s with 3:50s. Now the speed-o is still off reads about 65 MPH when doing 45 MPH. I have an FMX tranny. Does anyone know what speedometer gear I would need to bring this in closer to actual?

Thanks

 

[Cannoball888]

I don't fully understand it, but maybe this will help

http://www.mustangsandmore.com/ubb/speedogear.html

 

[72 fastback]

See what gear have in trans it is probally a 17 tooth gear try a 20 or 21 tooth gear. That should do the trick .I have 235 go r15 on my car with 3.70s in rear and it is only about 2 miles an hour fast.