It works how ever it's way off.

20=25
35=45
40=55

How do you fix that?

 

Don't know what year, model or transmission you have, but normally this can be cured by changing the small plastic gear where the speedo cable attaches to the transmission. The gears come in various number of teeth to make the speedo work for different axle ratios and tire sizes.

 

That part of the truck is stock, so it would be a stock transmission.

Where can you get the speedo gears at?
How do I determine what I have and what to get?

 

What year/model truck is this? Stock trans/rear end?

For an original 48 - 52, it's a lot less trouble to buy an adapter gearbox for the cable. You can get them in virtually any ratio. In your case, a 30% increaser would be about right. Speedometer Ratio Correction

I tried replacing the speedo gears in my 52's 3-sp because of a 3.50 rear axle. The problem is, none of the trucks back then had axle ratios that low, with the smaller tires common today, so there is no OEM combination that will get the correct speedo readings.

 

Sorry, 1950 Ford F-2

 

The gear is not plastic, it's nylon, but...

Unless the trans has been swapped, no FoMoCo vehicle prior to 1960 has a nylon speedometer DRIVEN (end of the cable) gear, as these were introduced in 1960.

Prior years, the DRIVEN gear could be cast iron or have a steel shaft w/a fiber gear riveted to it.

And even then, some 1960/72's could also have an iron or fiber gear, as it depends on the type of trans they have.

The fiber or cast iron gears have the Ford part number stamped on them (similar to this: 01Y-17271-E). Tooth count varies between 18-23 teeth, depending on rear axle ratio and tire size.

If the truck has P-Metric radials, the speedo will not read exact, as these types of tires are a different diameter than the original bias ply tires.

P-Metrics were not available until the 1980's.

btw: The Rating Plates used 1948/56 have the actual rear axle ratio stamped on them. Rating Plate located on the inside of the glove box door.