I have a 84 V35ll with a Kilometer speedometer and I wish to fit a MPH speedometer I know it will bolt straight on but do I have to fit a different ratio speedo drive to the front wheel .
Could anyone tell me what the ratio on the drive is for the Kilometer speedo and could anyone tell me what the ratio is for the MPH speedo drive
It is possible to work out the drive ratio. All to do with wheel circumference/rotation and road speed. The two speedos I suspect are the same internally and the face is changed. Another method would be to fit temporarily a push bike speedo. Then run the bike at 30mph and note where the needle is on the km speedo. Then repeat this for other important speed limits. When done mark it onto the old km speedo. You could take of the face and using DTP design your own face to match the other side. Print it out laminate it and stick it in the old case. This would be an interesting challenge.
All the best.
I took a chance on a straight swap, as the needle positions on the MPH speedo I found looked to be in the correct place on the original KPH speedometer. Fitted and took it out for a run using a GPS speedo as a check, and it was all good, no other changes needed. Have to admit a slight sense of amazement 
It does make sense that they are easy to swap, the different instruments are bought by the factory and bolt onto identical models whether they are for UK/USA or the rest of the world.
I could of course be wrong 
Yes far simpler to just have different dials.
I changed a kilos speedo to mph one for my Tonti, same thing.
Many thanks for the information guys
id buy a UK spec speedo head from Gutsibits or reboot and pop it in. much less hastle
That’s what he’s going to do, the question was does it need a different drive gear gubbins …
And the answer is, no. 
if not send it to speedy cables and they can clean it for you and fit a different face, although the mileometer with still register in Kms
An easier solution would be to fit a geared kph to mph convertor.
These are a small unit that simply fits into the back of the speedo, where the cable would go. Then the cable goes into that.
I got one years ago from MandP for a grey import I had. The speedo then reads in mph. You also get a “mph” sticker to go on the speedo glass.
Don’t know if they are still available but worth a try.
I have heard of these before too, not sure if you can still get them though.
Someone said these are now illegal so you will either need a new dial (Speedy Cables or other instrument restorers as noted above) or mark the face with whatever is to hand tape/paint/bird poo
I managed to find a decent MPH speedo head and replaced the old KPH head with it I checked the speed with other bikes and it register the correct speed and mileage .
Well done