My recently purchased mostly original 1945 half ton has a 100mph speedometer which appears way off. With it came a box with a 60 mph speedometer that shows the shift points very low, which suggests it has a 411 or 456 rear end "towing gears". I think this speedometer broke and was replaced with a 100 mph one that is wrong, and the original rusted in the poorly stored box because the truck has almost zero rust. It has an open diff and raising the right wheel only and turning the tire backwards 4 times spins the driveshaft 7 times. Or turning the tire 20 times the driveshaft turns exactly 34.5 times. I can't make the math work to match any gears that make sense. I might be able to make one good guage cluster out of the two, but what ratio is the rear, and which speedometer is correct? Thanks guys!

 

3.78 to 1

You need the 100 mph speedo

 

I get ≈ 3.45 to 1 and 3.5 to 1 from the description.
When jacking up only one rear wheel the differential spider gears double the turns of the axle ratio. To check axle ratio turn the single wheel twice and count driveshaft turns. In the checking above:
Wheel spun 4 time, driveshaft 7 times = wheel spun 2 times driveshaft spun 3.5 times -> ≈ 3.5 to 1 ratio
Wheel spun 20 times, driveshaft spun 34.5 times = wheel spun 2 times driveshaft spun 3.45 times -> ≈ 3.45 to 1 ratio

Fords if this era had three common axle ratios, 3.54 (least common, most desirable), 3.78 (by far the most most common), 4.11 (more often found in trucks than cars, least desirable).

It appears you have the desirable 3.54 ratio. Use the 100 mph speedometer. Page 632 of the Green Book gives the following parts to use with that axle ratio.
With 6.00-16 tires:
3 speed transmission use drive gear 21C 17285-C (8 teeth), 4 speed transmission use drive gear 01Y 17285-B (8 teeth), driven gear use 01Y 17271-D (21 teeth)
With 6.50-16, 7.00-16, 7.00-15, 7.50-15, or 9.00-13 (!!!) tires:
3 speed transmission use drive gear 21C 17285-C (8 teeth), 4 speed transmission use drive gear 01Y 17285-B (8 teeth), driven gear use 01Y 17271-C (20 teeth)

 

Wow guys, thanks for the great info. This is a pleasant surprise and now I know how to proceed. Just goes to show experience is very valuable.

 

hi, I had the same problem,I have a 100mph speedo. that with my 3 speed tranny shows 100mpg at a real 60mph, I found an adaptor thay plugs into the back of my 100mph speedo. and reduces the shown speed down to the correct speed. the adaptor was originally on the 100 mph speedometer ,but I had removed years ago and forgot about. there is one on ebay FYI.