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I'm trying to understand your question since the transmission has nothing to do with it. 78 has the version with the high beam indicator in the top. They come in 80 mph or 100 mph ranges.
Are you asking which speedometer drive gear you need? The number of teeth is determined by your trans, differential ratio and tire size.
Sorry, I did this post in haste. Yes I'm looking for the speedometer drive gear for a 4.56 32 inch tires C4. Trying to figure out which one I need and where to get it.
You will need to know the rear axle ratio and the number of teeth on the drive gear in the trans and the tire diameter to calculate the number of driven teeth on the speedo gear. I believe your C6 will have 7 drive teeth. But I'm not 100% certain since they came in a variety of configurations.
The formula for calculating the driven gear number of teeth on the speedo cable is as such
# DRIVE TEETH x AXLE RATIO x TIRE REV. PER MI. _____________________________________________ = # DRIVEN TEETH 1001
To calculate tire revs per mile
20168 _______________________________ = REVIOLUTIONS PER MILE
For a C4 the output shaft on transmission most likely is 7 tooth. Probably not going to get correct reading without a reducer, Ford used them on the cars with the low rear ratio's. Fox Mustang guys would try the aftermarket 23 tooth gears to correct for 3.73/4.10 gears. Ford didn't offer the 23 tooth and they have short life expectancy. If your tires are large enough diameter you might get halfway close with a 21, that just WAG.
For a C4 the output shaft on transmission most likely is 7 tooth. Probably not going to get correct reading without a reducer, Ford used them on the cars with the low rear ratio's. Fox Mustang guys would try the aftermarket 23 tooth gears to correct for 3.73/4.10 gears. Ford didn't offer the 23 tooth and they have short life expectancy. If your tires are large enough diameter you might get halfway close with a 21, that just WAG.
No 1973/79 F100/350 C4 or C6 has a replaceable speedometer DRIVE gear, because it's made as part of the output shaft!
C6: There are several different DRIVE gears "rolled on the output shaft," one has 6 teeth, the other has 9 teeth // C4 could also have 6 or 9 tooth DRIVE gears "rolled on the output shaft."
The nylon DRIVEN gears are replaceable, come with 16 thru 21 teeth, but the rear axle ratio and the tire size must be known to get the correct gear.
Pic: C6 .. Output Shaft is 7060 .. but there's no basic part number or picture of the DRIVE gear (it's 17285 w/a M/T), because it's made as part of the shaft.
NO C4/C6 replaceable speedometer DRIVE gears!!!
