This video from PUB 2009 will help visualize what's going on inside the speedo; in a nutshell, there isn't a mechanical/physical connection between the wheels and the needle. (modern electrnic speedos count pulses and a servo motor moves the needle accordingly, so they are typically more accurate since the number of pulses increase is linear)
There is an air gap in these older mechanical speedo systems, with a rotating magnet (on the speedo cable side) and a metal piece on the needle whose rotation is induced by the magnet's spinning, so you won't get a 1:1 match between speed (distance travelled) and what's indicated by the needle, and as you have seen, the error (or slip) increases with speed. This system would be terrible at registering Salt Flat Racer speeds...
This video from the Pagoda folks is quite helpful for visualizing the system:
If your speedo and transmission/rear axle are the correct ones for your car, and your tires are the size spec'ed for your car, then the error is likely "acceptable" for back in the day. Changing tire size (easiest for a prior owner to have done), or rear axle or trans tail housing (likely not done) would change the speedo reading, but not the amount of error, as the error is a function of that magnetic "drive" having slip.
Anyway, that's how I rationalize my older cars' speedo systems... Not sure if your 79 300SD car has a mechanical or electronic speedo.
03-22-2012, 09:28 PM
Linear Mode
