Speedometer data

Hello Richard - Have a look at the response from Phillip Hannam to a question on the correct speedo equipment  for an 88.  It's on the HeavyTwins forum entitled Dommie speedo drive gearbox raised by Tom McEwen,   2 weeks ago.  A 1600 is usual dependant on your back tyre and rim diameter (that determines your revolutions per mile) and speedo gearbox ratio.   All will be revealed for your set-up.  Good luck, howard

I was amazed at how much faster my 88 went after I fitted a pair of Commando clocks that I'd got at an autojumble.

Of course,I should have fitted a Commando 15:12 drive box too! I eventually did, and the bike slowed down to it's "normal" speed....

Richard - forgive me if you've worked it out yet but here's a link to  a copy of a query I answered from Mr McEwen three months ago in a similar vane:  Unfortunatley I can't get Paste fn working so we'll have to go back in time and history.  Go to Heavy Twins forum  then select Page 3 , scroll down to topic entitled Speedo drive box ratio on a 1957 99 raised by Tom McEwen on 8th October 2023.  Read my answer.  Cheers, Howard

Here it is!

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   

https://www.nortonownersclub.org/forum/speedo-drive-box-ratio-1957-99

Thanks Dan - How did you do that......regards, Howard