Ruairidh's suggestion is by far the straightforward and probably the most economical solution if it is magneto compatible, which I regret I do not know.
If you could contrive a drive from your DFL dynamo you would need to know at what rate it rotates with respect to engine speed. Then you would need a rev counter calibrated to that speed or, if yours was inappropriate, a little intermediate gearbox to convert the drive to suit your tacho.
If I were dead set on a mechanical drive I would try to contrive a drive from the front of the camshaft a la Gordon England Brooklands, probably using an appropriate tacho gearbox such as you can see if you scroll down here:
http://www.timelessmotorcycles.co.uk/html/drives.html
If you really wanted to use your gearbox speedo drive you could make a new face for your speedo. You would need to know the drive ratios in each gear, the final drive ratio and the appropriate tyre constant, then you could have concentric calibrated rings on your instrument face.
I wouldn't do this because I am lazy but I might follow 2cv practice and work out the speeds in each gear at the rev limit of your choice and mark the existing speedo face 1, 2, 3 (and 4 if you have such a gear) at the appropriate speeds.
Best phone Mr Mckenzie first, I think.
Regards,
Stuart
If you could contrive a drive from your DFL dynamo you would need to know at what rate it rotates with respect to engine speed. Then you would need a rev counter calibrated to that speed or, if yours was inappropriate, a little intermediate gearbox to convert the drive to suit your tacho.
If I were dead set on a mechanical drive I would try to contrive a drive from the front of the camshaft a la Gordon England Brooklands, probably using an appropriate tacho gearbox such as you can see if you scroll down here:
http://www.timelessmotorcycles.co.uk/html/drives.html
If you really wanted to use your gearbox speedo drive you could make a new face for your speedo. You would need to know the drive ratios in each gear, the final drive ratio and the appropriate tyre constant, then you could have concentric calibrated rings on your instrument face.
I wouldn't do this because I am lazy but I might follow 2cv practice and work out the speeds in each gear at the rev limit of your choice and mark the existing speedo face 1, 2, 3 (and 4 if you have such a gear) at the appropriate speeds.
Best phone Mr Mckenzie first, I think.
Regards,
Stuart