How do you calibrate or adjust the speedometer on a 1983 VW Rabbit GTI with a manual transmission?

Short Answer:

The speedometer cable drive gear, at the transmission end of the cable.

It is available in three different ratiOS. Each ratio is identified by color and is matched with the transmission code. A completely stock 1983 Rabbit GTi, with a transmission code of 4K, uses the red gear (VW P/N 171.957.821B).

Procedure:

Remove the speedometer cable at the transmission.

Remove the gear, held on by an internal spring clip and replace with one that corresponds to your transmission code or calculated final-drive ratio.

Make sure it seats on properly, you should feel a slight click. Give it a light tug to make sure it won't fall off. Then screw/bolt the clamp back down, You don't want the gear to come off and jam the transmission.

Things to consider:

The wheel and tire's rolling diameter will affect the accuracy of the speedometer. Tread wear and wheel/tire size are a couple of variables. The actual diameter of tires also vary between manufacturers, product lines and production runs.

Calculation of the final drive ratio against the wheel diameter will need to be done if you don't intend to maintain the stock rolling diameter of the tire when changing wheel/tire size.

If the transmission has been replaced, likely it's a different set of ratiOS. Sometimes this is intentional (to get better highway mileage, for example), but often it's not. The early GTi's stock gear ratiOS are among of the shortest that Volkswagen produced, some later diesels and fuel-economy variants, the longest. Each application requires a different drive gear to be accurate. Often, the relatively low-production GTi tranny gets replaced with a more common transmission, typically due to cost, availability of serviceable units or for the lack of a better understanding of the parts involved.

The mechanical speedometers themselves, as far I have experienced, are all geared the same. For example, a 1991 16v Jetta GLi speedometer will read properly in a 5-speed 1979 Scirocco, other factors aside, if the drive gear is matched correctly.

A binding cable and other damage or wear to other speedometer drive components may also need to be addressed.