How To: Ford CMP "Blitz" miscellany

[Jacques Reed]

Good Day,

While working on sorting out my speedo cable I had an idea that perhaps a Ford commercial truck speedometer driven gear might work in the single speed transfer case despite the Parts Manual identifying it as unique to CMP's.

I had recently sold an F-15 transmission and took photos of the speedometer gear used at the back of the transmission for the buyer. See first photo. If I could use a similar commercial gear then the drive cable end at the transfer case would be a simple formed square in the cable instead of a keyhole fitting which would make shortening the cable simpler.

The F-15 uses a 20 tooth driven gear but I could find no info on the number of teeth on a F-15A. So I removed the gear sleeve and driven gear to see what it looked like. See second photo. Surprisingly it is only a 15 tooth gear so it sure is unique to CMP's! Most Ford commercial truck speedometer gears are 18, 19, or 20 tooth. The F-15A gear is very different in other aspects also.
I then wondered why the big discrepancy in the number of teeth as an F-15 has a 6.66:1 diff and the F-15A a 6.50:1 diff. Very similar ratios and both use 16" wheels. Probably caused by the difference of number of teeth on the speedometer drive gear in the two vehicle types but I didn't have the number for a F-15.

I found a good device to determine number of teeth required for a speedometer gear on the Summit Racing website.
Inputting 4 teeth for drive gear- known from Parts Manual.
6.27 for diff ratio- (27/28 x 6.50 to allow for 27 teeth on driven gears in transfer case and 28 teeth on the idler where the speedometer gears are).
34"- for the tyre diameter as measured.

And Bingo 14.86 teeth!

Hope this is of some interest.

Cheers