Apologies Matt – ignore my comments on indicated speed (#15) …
ERROR
… a little over-excited there as in my case I considered stepping-up to a larger RR & thus taking-up the over-read tolerance & moving more towards the (illegal) speedo under-read. When in contrast you’ve moved (the opposite way) to a smaller RR & thus increased the speedo over-read – but as you’ve tested, your thinking is straight (despite confusing comments) & it’s not worth fretting over.
Apologies if I’ve added to any confusion. I think I’m right (?) in saying that a step-up in RR bring’s one closer to where indicated speed = actual (or true) speed - & a step-down in RR will increase the speedo over-read & thus widen the gap between indicated speed & actual (or true) speed.
I’d find it interesting to know, from those with SatNav kit indicating mph – what their SatNav mph reading is against the indicated speedo mph, & what tyre size they’re running.