19 replies to this topic

[Spud_133]

So what about the addition weight on the car when it is driving along, DILLIGAF?

I'd agree with using Coopermans method, as it is far easier, quicker and cheaper than it measure how far a wheel goes round.

[Guess-Works.com]

Afraid not he's right about the circumference thats all, you my as well wrapping a tailors tape measure around the wheel.........

Have you ever tried it ?

[Spud_133]

I have indeed, i simply meant the additional weight of the downforce, and my point is to show there are far too many variables to get an accurate and consise measurement.

[Cooperman]

Let me come back on here and try to clarify what I said.
The OP was looking for a way to compare the gearing change caused by going to different sized wheels and tyres and whether the resultant gearing change caused by the tyres/wheels would dictate a change in FDR.
Now, my method gives a quick and mathematical answer. The other way would be to buy the new wheels/tyres, fit them to the car, measure the actual rolling radius, then fit the originals and do the same. If the gearing was then not ideal, it would be necessary to remove engine and gearbox and change the FDR.
However, the mathematical calculation gives a nominal comparison which is quite close. we are not talking about speedo error here, just FDR comparisons.
What the mathematical calculation does is to enable a decision to be made without buying and fitting other wheels/tyres, then having to change the diff if the gearing is too far out.
For a Mini the differences between overall gearing with 10", 12" and 13" wheels and respective 70%, 60% and 50% profile tyres is not great, but I used to have to do calculations for Escorts and Datsuns with 13", 14" and 15" wheels and race, tarmac and forest tyres where the differences were quite significant and diff changes commonplace before events.
For speedometer and distance reading purposes, of course it's not accurate enough, if those readings are important, but as a gearing comparison it works well and that was the original question asked.

[cooperrodeo]

I have 10" wheels with 145 tyres, 3.44 diff and a speedo of 1280 turns/mile. I checked that 1280 turns really does increment the mileometer by exactly one mile. Arithmetic shows that for 1280 turns/mile and speedo gearing of 6/17, the car should go at 16.5mph/1000rpm in top gear. I was surprised then that a rolling road session reported only 15.65mph/1000rpm, even with new tyres.

The difference was accounted for by the measured effective radius of the tyre being only 230mm compared with 243mm demanded by the speedo. I have (expensive) Dunlop SP Sports and they barely reached 243mm radius unloaded, let alone loaded.

So, be wary of using these calculators you find on the internet. They do not seem to take into account the effective tyre radius when laden. My speedo read about 7% high on the bench, but with a further 5% indicated above (more with worn tyres) it is hardly surprising that some Mini owners overstate the speed their cars can achieve.