What is the RPM of the drive shafts with 165/70R10 at 110 km/h?
If it matters my diff is 3.1:1 (engine rpm is around 3300rpm).

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110k/hour = 1.833 k/ minute; a 165/70R10 diameter is close to 1.5m (should be rolling diameter to be accurate)
A 1.5 m diameter tyre has to turn 1,222 times to cover 1.833km
Therefore at 110k/h, the wheel (and the driveshaft) are turning at 1,222 rpm.
That's my answer. But I could be wrong...

Yep it matters.... a 3.1:1 diff means the input pinion ...hence crankshaft turns 3.1 times for every revolution of the crown wheel/output.

A "normal" Mini gearbox has a 1:1 ratio in top and normal drop gears are also 1:1 so each crankshaft rev turns the diff pinion once.

So the drive shafts will turn once for every 3.1 crank revs.... and hence 3300 crank revs will rotate the drive shaft 1024.5 times....

I think it a bit optimistic to expect to do 110 at 3300.... I run 3.1 with 165 Yokos and I reckon on doing about 100 kays at 3300 (which agrees pretty well with the theoretical 18.1 mph/thousand revs outcome for a 3.1 diff).

Cheers, Ian

The diameter of the wheel determines the driveshaft speed for a given overall kph - not the final drive. The final drive will make a difference to the RPM

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1.5m diameter Bill?

Circumference is 1524mm, about 1.5m

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YIK, Timmy.

Ummm i suppose semantically true ..but the actual rolling radius + diff ratio will (mainly) determine the engine revs at a particular speed. So its all related.... knowing one parameter you can calculate another..... However as we don’t know what kind of tyre and wheel we have its all a bit of academic.

I’ll go with my estimate at 100 Kay (based on measurements with similar ... but maybe not the same specs) you can extrapolate from there...

Cheers, Ian

For my calculations, I measured the "circumference" as 1.5m - (not the "diameter" as I wrongly stated in my first post) - and well picked up by David E. I said that I could be wrong!
That circumference was from a set of Dunlop 165/70/10 tyres in my garage, measured with a tape measure at about 1510mm. However I rounded it to 1500mm (1.5m) for the sake of the argument. Different tyres will have different circumferences and the real factor is the rolling circumference, measured on the car.

The original question asked for the driveshaft rpm at 110k/h on 10 inch wheels and tyres.
Answer: 1,222rpm approx.

The final drive ratio, engine rpm, etc., are not relevant to answering this question.

This link will provide the circumference and the revolutions per kilometre.
My calls show 1204.5 rpm, however, if you need to be really accurate you need the rolling radius at the pressure you are going to run the tyre at.
But that’s being a bit pedantic.

https://www.tyresizecalculator.com/tyre ... dimensions

Phil

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Good job boys, I did try to find a calculator on the net but I wasn't exactly sure what I was meant to search for (I kept getting tyre to engine rpm calculations which isn't what I needed). So a ballpark around 1200rpm at the drive shaft is good for me.
A follow up question is; Is there any chance the drive shaft will spin faster than that in a lower gear but at the same km/h? (say 3rd gear at 110km/h, or any gear at redline 6500rpm).

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No, the driveshaft RPM is dependant entirely on road speed.

You will get a very tiny variation between both driveshafts as they spin at different speeds when you go around corners, or during wheel spin.

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Yeah I didn't think that would be the case but thought I'd check.
Thanks.

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