And we're still interested.
Yes, previous twit put 20" Stormer copies on there, with (IIRC) 275/45R20s on them. I measured and they come out at the right rolling radius so shouldn't affect more than +/- 1%. And I'd expect consistent error throughout the range.
I'd originally just assumed that it was slow throughout and by the same %age / margin, but obviously it has a mind of it's own! And if it was slow by around 25% first thing I would have been looking for was a loose / excessive gap / missing sensor meaning it was only reading 3 wheels.
There is a slight ABS / brake error which I hadn't had time to search for, but I think that will be to do with the powered side from the way it behaves. Shouldn't affect speed count.
ABS / brake warning lights behave correctly at ignition / set off from start. But they both illuminate for a second or 2 when first applying brakes, then go out again. As fluid level is good, I was guessing that was some failure on the power side and then clears as the pressure from the pedal / servo builds above a level. Typically it doesn't match any of the behaviours listed for the system in RAVE!