Well I came across a post on Iroll where a member had wired in a trim pot in place of the resistor, to allow for more calibration possibilities.
Thought I'd give it a go myself.
Unfortunately I didn't take progress pics, as I was doubtful I was going to succeed. ( A previous attempt to desolder a component from a PCB and solder on a new one did not go well at all) My luck must have been better this time.
After playing around with the trim pot, id say the Speedo is about 98% accurate, it seems to be spot on at a particular speed then slightly off at other speeds, though it's much better than before.
Worth a go if the speedometer on your 240 is significantly out like mine was.