Correct speedo drive for an 18" wheel

Previously Chris Grimmett wrote:

The usual idea is to fit a fatter profile 18" rear tyre, bringing the rolling diameter back to the nominal 26" and retaining the original speedometer drive gearbox with its 15:12 ratio.

You will need to look at the diameter of your rim/tyre setup, do the maths, then work out what ratio of speedometer drive you would need to get the correct reading. I do not know what ratios came with the Dominator items. You have not indicated at what road speed the 10 mph offset is indicated - it will not be a constant, but speed proportional.

You could ignore the whole thing and fit a bicycle speedometer unit which gives a more accurate reading as you can calibrate it.

Even better, but more expensive, fit one of the new electronic Smiths speedos.

Thanks Chris. The electronic unit has crossed my mind. A second hand Dommi unit will be cheaper than either a new tyre or an eletronic clock so i may just give it a go. I will let you know the outcome. Mark.

Previously nicholas_marshall wrote:

all you need to do is drill a small hole in the back of the speedo where the adjuster screw is and adjust untill the clock is spot on,i did this on my commando with a sat nav strapped to the handlebars ,i got the location of the hole from the "magnetic speedo repair manual" regards nick

Thank you Nick. Thats perfect. I will give it a go. The Dominator drive, for the record, only makes matters worse.

Mark

Previously david_evans wrote:

There's a free app for android mobile phones called android speedo. If you are prepared to strap your mobile phone on your handlebars it will give you a good idea how far you are out. I've got 18" wheels on my Commando, the rolling dia is about the same as it was with 19", not worth doing anything about it in my opinion. when the speedo is driven from the rear wheel, it's just another dial giving you the revs (providing the clutch is not slipping it should be constant) that's why most bikes have the speedo driven from the front wheel

Thanks David. As you say its not the end of the world, i had a mate signal me at varying speeds as to get a guide on what was indicated so ive a good idea what speed i'm doing but it would be nice to have it right. Thanks again Mark.