I have a bone stock 1949 F1 which is totally stock and basically unrestored. The speedometer reads about 10 mph slow at around 50 mph - it shows 50 when the truck is actually going about 40.

It has a stock three speed manual transmission and Dana 41 rear end. I am not sure what the gear ratio is in the Dana, but it seems like it is the stock 3.78. The truck can get up to about 55-60mph top speed, which is about normal I would think.

The speedometer needle does not shake at all - it is very steady. I did have to replace the speedometer cable (the old one broke in half just a few inches from the transmission).

Any ideas on what to do?

 

maybe the speedo gear is for a 3.92 rear end? you can check into the speedo gear and get one of a different tooth count.

 

If it is reading slow, do I need more teeth or fewer?

 

more, i believe, the gear that you change would be the driven gear.

 

You can put smaller tires on it too!

 

The way you describe it the speedo is fast , not slow. You said the speedo shows 50 when the truck is doing 40. You would need more teeth to slow down the speedo .

 

There is a page near the back of the parts book that will give the tooth count needed given your rear gear and tire size. The new ones are plastic but available in several places.

 

when you dig into this, this driven gear is on the rear of the transmission. the speedo cable threads into it. once you unhook the cable, there is a nut that holds the gear in place. it is far easier to change it if you have the tranny cover (in the cab) off the tranny if that is possible. i have a light duty 3 speed, my gear was metal, i don't know if mine can be replaced with a plastic gear or not.

 

Since I'd assume none of the gears have been changed, it's the speedo. There is a big clock spring inside that calibrates the needle's position. A speedo shop can adjust it.

Changing gears doesn't work very well. Been there, tried that. I tried to compensate for the 3.50 rear end in mine, by using the chart to find what gear would correct it. It chewed up the new gear in 10 miles. There are distinct combinations that work, and none of them used on the F1 trans went below the 3.73 ratio used on 48-50 F1's.

The plastic gears are a completely different type and won't work.

 

What size tires do you have on the rear of the truck - That is the final determination for vehicle speed -v- engine rpm