The V8 Owners Forum -

Thought that page from Rimmer would explain - but it doesn't. There are two drive gears used on the SD1. And very possibly others on different versions - I dunno. Although one is far more common on the SD1 than the other.

But the Rimmer page does give the teeth number of the drives used on the SD1 covering rear axle ratios from 3.45, 3.08 and 2.85 on the same drive gear. So if you count yours, it's possible there is one which might do, and easier that changing the drive gear inside the box.

From what I've found so far there are (were) 6 output gears ranging from 20-25 teeth in colours
Orange
Green
Red
Black
Yellow
White
Each will give around 5% speed difference over its neighbours, if I'm lucky ill currently have one in the Black-White range AND (if im double lucky) I can find one 3 teeth smaller then I should be good.
Failing that I'll be on the lookout for an in line cable speed conversion unit or look at getting the speedo re-calibrated

The one thing I won't be doing is taking the gearbox apart to change the internal gear .

One thing I did dream about a while ago was the idea of creating a circuit to drive a stepper motor via a pulse output from a gearbox sender with variable output.
As I've been investigating this I've found that others seem to have had similar ideas although I've yet to find one which is digital (and hence very controllable) they all seem to be analogue. Going to be cheaper to re-call the speedo though I think.