1955 Ford speedometer reads too fast??

[KULTULZ]

Originally Posted by steve fritz

ok , heres the deal...

original 1955 Ford Ranch Wagon 6 cyl , 3 speed O.D.
Had 47/11 ring and pinion , which is 4:27 ratio
replacement tires are 205R 75-15.

Speedometer reads approx 10 mph too fast with original 22 tooth speedometer cable gear.

Here is what I found. There is a conversion chart(s) for calculating needed driven gears. The only problem is finding tire specs for older tires.

Supposedly, the car came through with 6.70 X 15 which has been replaced by 205R-75-15. The diameter of the original bias ply is 28.58" while the diameter of the replacement residual is 27.1". That is enough to throw the speedometer off.

FYI - The driving gear is 7T. The supplied driven gear for your application was 22T.

When you use a speedometer calculator, it call for 22T (B5A 17322-D - BLACK) with the updated tire size. This was also used for assembly with the bias tire. So FORD averaged out speedo gears.

For reference, here are two different gears in the same gear series -

21T - B5A 17322-B (GREEN)
23T - MC 17322-B (YELLOW)

When you decide on the needed gear, use the mile markers on Interstate to confirm accuracy.