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My 250 is wearing 17/42 sprockets , i think this is "export gearing" ? , I calculate that this should equate to 99 mph at 8500 rpm , seems like a good compromise , rear tyre has a dia of 24 and a half inches or 76.979 inches circ. Are my maths somewhere near?.

" My 250 is wearing 17/42 sprockets , "

____ Having never owned a Mach-I, I can't be positive of what gearing that model came with but, I think it was the same as the later-1965 '250Mark3' came with, which was 40/18t,, (so as to be able to obtain the top-speed of 110-MPH @ red-line).
__ The pre-1965 'Diana Mark III' models came with 43/18t, (and hadn't been able to claim the 110-MPH top-speed).


" I calculate that this should equate to 99 mph at 8500 rpm , seems like a good compromise "

____ The Monza came with 45/17t, for a relatively low sprocket-ratio of 2.65:1,
and the GT came with 45/18t -(2.5:1) gearing which was pretty-much right in between that of the Monza's & a Diana Mark-III's -(2.39:1),
and since your 42-17t -(2.47:1) sprocket-ratio is pretty-much the same as a GT's, then your gearing could indeed be considered as a good compromise between the Monza's & the Diana Mark-III's, sprocket-ratios, (especially since it's also a tad closer towards the very-high Mark-3/M.1 -[2.22:1] sprocket-ratio).


" rear tyre has a dia of 24 and a half inches or 76.979 inches circ. Are my maths somewhere near?. "

____ Well actually, your rear-wheel's 'diameter' is not quite exactly useful for calculating the most accurate figures... Rather, you ought to figure-in the wheel's 'radius'.
__ Have you somehow missed the related thread-posts which fairly well covered this topic-matter, posted just last month ? ... viewtopic.php?f=3&t=564&start=10

DewCatTea-Bob wrote:If you're interested to know your wheel's actual radius, then simply dab a small wet-spot onto your rear-tire's tread (or insert a little-piece of chalk or crayon into it), and then let me know the distance measured between two spots transferred onto the ground -(smooth pavement), from your straight-line rolled wheel.
And with that distance-data, I'll then demonstrate how to use my-own formula for calculating speed @ RPM (or calculating RPM @ a given speed).

" Are my maths somewhere near?. "

____ Well without factoring-in the actual rolling radius of your rear-wheel, and we simply go-ahead & assume that your tire's running-circumference is really 77-inches...
Then to 'check' your results (of 99-MPH @ 8500-RPM), we take the number of inches in a mile (63360) and divide by the 77-inches, to get 822.86 rotations (of the wheel per mile), then multiply by the 99-MPH & your-particular overall gear-ratio of 5.98 -(60/24 x 30/31 x 42/17), then divide by 60 -(min. per hour), to thus then get a corresponding RPM of just 8115 !
__ But likely your math is off by more than just the 385-revs, as the (omitted) factor of 'tire-growth' would tend to lower the revs still further, (possibly up to as much as around 8%, depending on the tire's air-pressure).
So it could possibly be that your actual revs would be closer to 7500-RPM (near 99-MPH).


Fun-Cheers,
DCT-Bob

Sounds like I could be overgeared, the rear tyre is a newish Pirelli MT65 3.00 by 18. Have to do a circumference measurement.

The distance measured between 2 paint spots transferred to the ground would be the tires circumference, which you can also get by (Pi*Diameter).

" Sounds like I could be overgeared, "

____ Over-geared for what exactly?
With 42/17t, I rather doubt it, unless the majority of your riding is within city-limits.


" the rear tyre is a newish Pirelli MT65 3.00 by 18. "

____ Tire-size does make a considerable difference, and at 77-inches, I had assumed that you had a slightly larger size mounted.
__ I think n-c Mk3 models had a 18x2.75" rear-tire with about a 75.5-inch (normal/unladen) circumference, and about a 11.5-inch 'working-radius', (but I don't recall exactly and these figures are just guesswork based on my past related experience.)


" Have to do a circumference measurement. "

____ Yes, an actual-measurement of the 'working-circumference' would be extra useful...
However you could instead measure the 'working-radius', for calculating from...
To get that pertinent figure, first make sure the tire-pressure is set correctly and then you could have someone measure from the center of the rear-wheel's axle directly-down to the ground-surface, while you're fully sitting upon your Duke. _ That way, you should get the actual 'working-radius' , which is considerably more accurate to work with than merely the (supposed) 'diameter' of the wheel. _ However for this radius-measurement, fairly more accurate readings (down to 1/16th") are required, compared against fully measuring the actual working-circumference. _ And to avoid having to hold the axle exactly level with the ground's surface, you can lean the handlebar-end against a wall, and then get the average axle-height by measuring down from both ends of the axle.
Then multiplying the working-radius by '2Pi' -(6.2832) will then provide the 'working-circumference', which is needed in order to go-ahead & calculate any revolutions per time-period results.
__ Then to the working-radius, we ought apply the 'tire-growth factor', so as to get corrected figures, as speed increases.
This often overlooked factor is not worth bothering-with at speeds nearing 60-MPH, if ya have the wheel's NORMAL/unladen circumference (which is longer than the 'working-circumference'), to work with. _ That's because, while the normal-radius is flattened-down near a rolling-stop (thus then at the 'working-radius'), by the time the wheel's rotational-speed has reached 60-MPH, the flatted-radius has been pretty-much canceled-out back to normal. _ And fairly well above 60-MPH is where the tire-growth factor tends to have most pertinent effect, which certainly alters otherwise good standard math-calculations.
Not calculating-in this factor, can possibly vary calculation-results by as much as 500-RPM, which in my opinion is unacceptable.
I'll be happy to review my-own formula for this RPM @ speed question, as soon as I'm supplied with a needed rear-wheel working-figure (either radius or circumference).

UPDATE: - It has since occurred to me that since it seems you now realize that you need to actually measure your rear-wheel's actual working-circumference (obtained by rolling the wheel while fully loaded), then it must be that you surmise your previously given figure of 24.5-inches for your rear-wheel's diameter is merely it's normal/unladen diameter, which is not precise enough for accurate calculations.
So now if I go-ahead & conclude that that's the actual case, then I can apply the 'tire-growth factor' in reverse, (to your wheel's normal -[as if lifted on center-stand] dimensions), so as to obtain a more likely working-radius for your rear-wheel. ...
__ So, here follows the extended math for all this...
The 'normal-diameter' of 24.5 x Pi = 77-inches, and to that I apply the INVERSE of the tire-growth factor, thus obtaining a relatively likely working-circumference of 74.5-inches.
__ Now with that unusual math figured, we can next get back to (more accurately) dealing with the original question of actual RPM @ 99-MPH...
To get the working-radius, we divide 74.5 by '2Pi' which equals 11.85-inches.
To that working-radius I apply the tire-growth factor at given speed -(99/Pi sqrt, + 1), which then equals 12.5-inches.
To next get the expected running-circumference at given-speed, we then multiply the resulted 12.5 by 2Pi, which equals: 78.6-inches.
Now we figure that that means the rear-wheel will turn 806 revs in 1-mile.
Next to convert that to engine-RPM, we multiply that result by the given-speed of 99-MPH, then by the OGR, & then divide by 60 (to convert from per-hour to per-minute) ...
So in this case, it's 806 x 99 x 5.98 /60, which thus equals 7950-RPM.
This t.g-factor corrected figure compared to the result achieved in my previous post, indicates that the spinning tire at 99-MPH cuts RPM by about 150-RPM, an amount worth adding to the 385-revs which has already been in question.
__ So anyhow, it's pretty-safe to conclude that with your current sprocket-gearing, when your tach shows 8k-RPM, your speedo should then read 100-MPH. _ Fairly neat gearing to have, I'd say !


Fun-Cheers,
DCT-Bob