|Most speedometer errors are proportional to the car's actual speed. A fixed error, such as a constant 20 mph regardless of speed, pretty much has to be a fault in the speedometer itself.
First, verify that a PO has not installed an incorrect speedometer. The proper part for a US-market car is a Smiths SN-5230/00 1000 (sometimes marked SNT-5230/00 1000), which will be visible along the bottom of the speedo dial (see attached photo). The 1000 indicates that the speedo is scaled for 1000 cable revolutions per mile. Since one mile per minute is 60 miles an hour, if the cable rotates 1000 times in one minute, the speedo should indicate 60 mph. You can test this if you have, say, a drill press that turns at a known rotational speed -- simply insert the transmission end of the speedo cable into the drill chuck and activate the drill.
If your speedometer calibration is off, a speedo repair shop should be able to correct it for a reasonable fee. If the speedo is broken, finding a (hopefully functional) used replacement on eBay is probably the most cost-effective approach. Bear in mind that replacing a speedo generally raises some minor legal issues if/when the car is sold.
IIRC, the 205-series tires often used on JHs are about 2% smaller in diameter than the original 185 series tires, so that when the former are installed the speedo will read 2% over the actual speed. The 4-speed rear end gear ratio is 3.727 while the 5-speed rear end is 3.45, so if a PO has swapped transmissions or rear ends (but not both) you could have an 8% reading error, the direction depending on what exactly was changed. Finally, while I'm not familiar with the speedo drive gear ratios available for either the 4- or 5-speed transmissions, but installation of the wrong gear might conceivably give an error of perhaps as much as 20%.
If you'll add your location to your profile, it's possible that a JH owner near you can recommend a trustworthy speedo shop or perhaps just the ones to avoid.
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