Which is more inaccurate?

[Sibbo]

I'm pretty sure my speedo is inaccurate but I thought a tacho was always right ? If it is from Barry's chart I was doing 115mph !

[barryh]

Someone might have changed the final drive ratio without re-stamping it.

You could rotate the rear wheel while observing the number of rotations of both the drive shaft and rear wheel. Perhaps through the the filler plug aperture you could somehow mark the part of the driveshaft visible through the hole and count the number of wheel rotations to complete say 10 rotations of the shaft then work out the ratio.

[Sibbo]

Thanks mate, I'll do that cos the numbers don't really add up.

[Sibbo]

7.8 rotations of the rear wheel gave 28 rotations of the drive shaft ..... approximately 3.6 to 1.

Is that a standard ratio? It seems close to a 60/6 if my memory serves.

If so does anyone have a chart that gives mph or kmh per 1000 rpm with that ratio final drive ?

[Bamboo812]

Don't know if this will help: http://www.largiader.com/articles/gearing/

[Sibbo]

I can't make that one combine a 5 speed box and a 3.56 final drive ratio , is it just me ?

OK, a bit of arithmetic . 3.36 divided by 3.56 = .944 x (Barry's chart )115 mph = 108.5 mph.That sounds about right to me .

Thanks all.

[barryh]

7.8 rotations of the rear wheel gave 28 rotations of the drive shaft ..... approximately 3.6 to 1.

Is that a standard ratio? It seems close to a 60/6 if my memory serves.

If so does anyone have a chart that gives mph or kmh per 1000 rpm with that ratio final drive ?

Nearest would be 32/9 or 3.56.

Taking account of the 1.5 reduction in the gearbox that makes an overall ratio of 5.34

Here the chart recalculated for a 3.56

[Sibbo]

Thanks Barry, that confirms my arithmetic .

[ME 109]

Thanks Barry, that confirms my arithmetic .

I often said to Charlie that I reckoned you had arithmetic.

[Sibbo]

I have this really good cream that relieves it but.