An old style mechanical speedo is a 2 part instrument, an odometer and the speedometer. The odometer is purely mechanical with a constant ratio between the input cable and the mileage drums. The input cable also spins a magnetic drum which then magnetically pulls a flywheel which has the speedo needle afixed to it around.
You can play around with the odometer part by swapping the end gearwheel and pawl for gearwheels with a different number of teeth - but - not a lot of point really. The distance between one town stays the same whatever the instrument reads.
The magnetically driven drum isn't capable of re or de-magnetising to reset the speedos calibration.
You can however reset the instrument for the most critical speed for yourself.
Drive the car against your gps at that specific speed and note the instrument reading.
Remove the speedo cable and then the instrument from the car. Ease the peened over bezel mounting tags, twist the bezel and remove. Release the mounting screws on the back of the instrument to allow the removal of the complete innards. Use a finger to rotate the needle to the indicated speed that you know the true speed of and hold it there. Mark a pencil line on the flywheel against a fixed part of the cage behind the dial. Release the needle and then prize the needle off, using a pair of tea spoons as levers. Rotate the flywheel to your register point and then re-affix the needle showing the correct gps speed.
Thus the instrument is recalibrated, albeit to be 100% accurate at only one speed, but you know which one, and which other speeds either side are safe or licence endangering. Sounds crude but it works and give reasonable level of comfort.
Reassembly is in the reverse order.
Smiths, in their own service literature, recommend cleaning and re-greasing (not oiling) the cable every 10,000 miles, probably worth pulling the cable inner out and doing that whilst you're poking around under the dash.
