3.07 2.88 CWP

[mgcjag]

Hi Colin...so are you actually saying that speedometer reading and distance traveled will be the same for a fully inflated tyre and a half inflated tyre.....Steve

[Series1 Stu]

None of these calculations are truly accurate but they do give a good indication when comparing tyre/wheel combinations.

If you're really pedantic then you'll need to find a way to account for tyre diameter increase with centripetal force as revs increase.



Regards

[lowact]

Can't say absolutely, imo tyre pressure would need to be unchanged for absolutely no change to speedo. I'm considering that rolling radius, due to the flattened bit on the bottom of the tyre, is caused by the weight of the vehicle, with tyres at recommended pressure.

Low tyre pressure would increase the flattened bit but that's not "rolling radius" its a flat tyre. In any case with car tyres is mostly due to flexing of the sidewalls, the tread is very much stiffer so even with flat tyres the change in perimeter, if anything, would be very much less than you might calculate based on the flattened height of the tyre.

Where tyre pressure does feature is in calculation of the rolling resistance which, along with aerodynamic drag, is used for predicting minimum torque. I'm using 2.5 bar.

[mgcjag]

Online calculator for anyone wishing to enter their own data.....Steve https://wahiduddin.net/calc/calc_speed_rpm.htm

[Fspp369]

Oh dear!
Boys will be boys!,,

[PeterCrespin]

I’ll see your pedantic and raise you one… that is sometimes done but imo is wrong, distance travelled is the tyre perimeter. We only use the radius to calculate the perimeter, which doesn’t change no matter how much the tyre is squashed. So the correct calculation is actually the easier one ...

Nope. Not buying that Colin - Steve’s right. Everyone can have an ‘imo’ but the facts are objective, not subjective. Your opinion is only correct for a solid wheel (imo, of course :-). Although the tread belt doesn’t change length it also isn’t centred on the hub so the axle/vehicle does not move by the exact distance of the circumference, it moves a distance equal to the circumference calculated by Pi x twice the rolling radius.

Hard to demo unless you mount justan easily-compressed tube on a spare wheel.

EDIT: I wrote the above when your quoted post was the latest one. It may repeat what others have said since.