[brian_]

Here is another option http://www.hukebasart.nl/huke/?Elektrische_snelheidsmeter or the English page http://www.hukebasart.nl/huke/en/?Electronic_speedometer

The English page is missing the description, but Google can translate the Dutch page.

This is interesting but I can't quite figure it out.

Based on the instructions, this appears to use a set of magnets strapped around the propeller shaft (a.k.a driveshaft... you would have to use the inner CV joint on one axle shaft), and a magnetic pickup ("pulse generator") to produce the electronic pulses needed to control the motor which rotates the speedo cable. The translated Dutch description and photo confirm that.

 

[brian_]

Not the most elegant thing but quite usable and a good idea.

For someone without an electrical pulse source, this is about as direct as is reasonably possible... but I agree, the whole system is not as elegant one would like due to the need to go from a rotating shaft to an electronic signal and back to rotating shaft.

$300, hmmmm

Seems like a lot, but not surprising due to the number of components in this approach... and it is a specialty item.