Greetings, recently purchased a 1960 F100 long bed and learned (when we pulled the speedo cable) the DRIVEN GEAR is missing. Trans "A" (3 on Tree); Axle 01 "3.70" currently running 235-70-15 tires. Any idea how many teeth I need in it's replacement?

 

What is the wheelbase? There are THREE possibilities:16T, 18T; 19T .. The 16T nylon gear is a different design than the 18 & 19T nylon gears.

The problem is, the parts catalog only lists the original bias-ply tire sizes (scroll down to Speedometer Gear Chart in pic).

 

118" long bed with a 292 motor.

 

Funny, none of the catalogs go near a 16 tooth gear for this.
Tire Rack has a conversion chart.
6.70-15 shows 195R15 or P205/75R15 or 215/70R15 or 235/60R15
7.10-15 shows 205R15 or P215/75R15 or 225/70R15 or 235/65R15 or 245/60R15 or 265/50R15

 

The easiest thing to do is to calculate what will need with your current choice of tires.

The formula for calculating the driven gear number of teeth on the speedo cable is as such
You will need to know the number of drive teeth in the trans though.

# DRIVE TEETH x AXLE RATIO x TIRE REV. PER MI.
_____________________________________________ = # DRIVEN TEETH
1001

To calculate tire revs per mile

20168
_______________________________ = REVIOLUTIONS PER MILE
TIRE DIAMETER (in inches)


Example
A drive drive gear with 7 teeth and a tire diameter of 28.89


20168 / 28.89= 698.096 And round to 698 Revs per mile.


​​​​​​​So knowing the revs per mile we can use the other formula

7 x 3.92 x 698 / 1001 = 19.13 And to round it to 19 So you need a driven gear with 19 teeth.

 

A 6.70-15 tire has an overall diameter of about 28.5" the 7.10-15's Range in size from 28 to 29"
The ideal replacement for the 6.70x15 is the 215 80 R15 but they are unobtanium in North America, but easily had everywhere else on the planet.

Do not listen to many of the conversion charts on line they are very wrong.