Speedo drive box ratio on a 1957 99

There are two commonly found on Dommies,  2:1, which normally goes with the magnetic type clocks, in my experience, and which it sounds as if you have, and 15:12 (which I would call 5:4), I think that the NOC shop has both.

hello Tom - As Niall correctly says there are two rear wheel speedometer gearboxes utilised on Dominators.  From 1964 magnetic instruments from Smiths were used and two types were possible indicated by the number 1600 or 1000 on the face.  This indicates the number of input turns per mile. With a  19 inch rear wheel and an approximate diameter of 26 inches this would give approximately 800 revolutions per mile so with an instrument marked as 1600 a speedometer gearbox of 2:1 ratio is required. Alternatively if an instrument with 1000 is used then a gearbox with a ratio of 1.25:1 is required.  Smiths Ltd in their wisdom indicates this ratio as 15:12.  The usual instrument for a 1957 Dominator would have been a chronometric  one and they are calibrated at 1624 revs per mile so a gearbox with a 2:1 ratio was fitted.  The problem comes when a tyre size is fitted that gives quite a different revolutions per mile but that's another query. Cheers, howard