OK, out with the calculator, or pencil and back of an envelope.
Now, knowing your overall gear ratio - say 5:1 and working with say 1000 engine rpms in top gear
(1:1 in the gearbox, conveniently)
the back wheel will turn 1000 divided by 5 in one minute = 200 rpms.
If your back wheel is say 80 inches around, and doing 200 rpms, it does x feet min, which is y mph.
I figured out my dommie does 1264 ft min per 1000 rpms = 14.4 mph
which neatly means its doing 100.8 mph* @ 7000 rpms. (14.4 x 7).
And, the top gear in an iron dommie is 4.78 : 1 ( think ?)
I rather suspect I've used a fairly small sized rear tyre too in these calcs, which may make them a bit low-ish.
I think, the page of calculations with this little chart is missing, so I'm reconstructing this from memory.
I don't believe I've ever seen this in a formula anywhere, although it would be simple enough to do.
*Except an iron dommie won't ever do 7000 rpms in top, so 85 to maybe 90 on a good day @ ~6000 rpms.
And Smiths speedos aren't generally that accurate, unless they have been especially calibrated.
Its worth commenting also that different brands of tyres are different sizes (diams) although not by much,
and tyre pressures and tyre wear alter the numbers, although not by much,
and even rider weight does alter things a little.
So the numbers are always going to be +/- a little bit, maybe a few to 5 percent even.
And another possibly 5% at least for the Smiths, so thats maybe +/- 10% accurate, mostly ??
I've been looking to add a tacho to a dommie, for many a year, and it may just happen 'sometime soon'.
Just out of curiousity, to confirm these numbers ??https://s17.postimg.org/lfs2zulq7/M7tim ... odrive.jpg