which speedo gear should i use?

[Falcon79]

I am switching my 68 coupe 6 cylinder car over to a v8. I bought a c4 to go with my 302. Should i use the speedo gear that came with the new transmission? or should i use the one from the 6 cylinder transmission? I know very little about this so if someone can give me an iea what to do that would be great!

Thanks

 

[latamud]

TCI - TECHNICAL INFORMATION: Speedometer Gears

Things that will affect what speedo gear to use:
rear tire diameter
gear ratio
drive gear

 

[Falcon79]

rear tire diameter: running 17x8 wheels from a 2003 GT
gear ratio: no sure but should be in the 2.79-3.20 range
drive gear: no idea

 

[2+2GT]

You HAVE to know how many teeth the drive gear has before you can proceed. With the speedo cable removed, put a dab of white lithium grease on the gear. Turn the shaft, counting the teeth, until you get back to the dab of grease.

 

[D.Hearne]

If the speedometer was dead on before the engine swap, it'll be dead on afterwards. Almost all Ford transmissions used the same number of teeth on the drive gear in the transmission.....7 teeth. The only ones that didn't had rear gear ratios in the 3.90-4.30 range. If you changed the tire diameter, then that is the only thing that will throw off the speedo reading. Going to a shorter tire requires more teeth on the driven gear (one on the end of the cable) Taller tires, requires fewer teeth on this gear.

 

[D.Hearne]

All C-4 transmissions have 7 drive teeth cut into the output shaft.