Don't assume it's the sending unit.
The speedos/odometers do fail.
Plenty of stuff on this board re. details of fixing them.
Do a search on "Speedometer".
Try the 700/900 FAQs, I'm not sure if it's there. Lots of that material applies to 240's.

Before doing much else, check the connection on the back of the speedometer.
There's a set of 3 wires that connects via one terminal.
There's a retainer clip that usually gets lost after the first time it's removed.

When messing around back there, be sure you DON'T hook up the red and white wire that's hanging around behind the cluster, unless you have a tachometer.
The red+white wire carries a high-voltage signal for the tach and will fry other stuff if you hook it to anything else. Doing so cost me a speedo and a distributor. Not just the distr. cap - the distributor got fried.