Mike I dont think there is anything to tweak in the speedo they just work on the number of turns the cable does, you will need a little gearbox to change the speed of the cable. Do they check the speedo on the test? they don't here on the MOT just record the mileage. If all else fails you could fit a bike speedo, I have one on mine, works fine up to about 60, records all sorts of things and costs about £10.
Peter