Speedo reading

[limeyrob]

Yes, me too.  What this discussion has done is made me find out what it is out there as my bitza came with 1 wrong Chronometric and no tacho which in the worst case could be £600 on a bike that cost under £3k which screws up my budget.  As its a bitza ("Special" to you )  I have a bit of leeway and there's clealry a lot more out there than I knew about.  These speed transducers made for BSA gearbox drives look interesting and I'm sure one could be used on an engine rev drive too.  OK they're £42 each but now I know about them I can search for something similar but cheaper.
https://www.digital-speedos.co.uk/speed-sensors-c9/classic-british-bikes-gearbox-adapter-p470
OK its a PR advertorial, but there's some useful info:
https://www.oldbikemart.co.uk/digital-speedometers-we-asked-an-expert/

[Rex]

If you have a Chrono (even a "wrong" one) stick it on and live with it. You're not going to get pulled doing 100 on the motorway, and just keep up with the other traffic around town.
Or, if the wrong one actually works, get a mate to follow you to see how far out it is, and ride accordingly in future.
And that's if you even see a copper these days.

[limeyrob]

Ha, yes, but I have form.  I also drive a 1970 Series Land Rover and because its got a different diff and wheels the speedo under reads by 10-15%.  So I forget one day and go through a speed camera at 33 mph.   I have to do a "speed awareness" course at a cost of £95 and spend a day in a classroom full of equally p**d off people.  What's doubly annoying is that I seldom go over 45 in the Landy .  I now rely on the sat nav but one vehicle like this is enough

[Slymo]

If the needle is bouncing it's a magnetic probably Indian reproduction.  The Chronometric speedos àre infact the acme of instruments and are increasingly valuable. Chronos tick up and down the scale and have both odometer and trip windows whereas the Indian knockoffs have just the one set of numbers. The original Chronometric gauges are very accurate as long as they are clean and the drive is the appropriate ratio. This is often written on the face. BSA A10s with a gearbox drive are often 1440 turns per mile. Wheel driven gearboxes are quite often 1600 turns per mile which relates to 2:1 at the gearbox on  19" wheel. There are a wide range of variations specific to the machines they are intended for. Magnetics are a way cheaper instrument to make and are OK as long as you understand that they are only likely to be accurate at one speed as they work against a hair spring. I generally set them up to be right at 60mph as it's often the middle of the gauge and a fairly crucial speed for staying clear of the law. The chronos can be adjusted to work within a reasonable range by removal or addition of clip on weights on the oscillating pinion (?). This is fairly delicate work and wants to be done by someone with appropriate skills (I have a clever pal). Otherwise as suggested above just learn what it's telling you and keep your eye out at the swap meets for the appropriate gauge.

[Triton Thrasher]

If the object is to know your speed, GPS speedos are available.

[Craigy]

It was more about having the correct speedo. All sorted now. Thanks