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.
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.
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