One for the mathematicians

[DavidB]

Currently I run 17” wheels with Longstone tyres - 652 mm diameter. At 5500 rpm this equates to 73 mph.
If I switched to 19” wheels and 669 mm tyres, what speed would this produce?
Am I simplifying things by assuming that as the difference is only 2.5% the speed would only increase to 75 mph or is there more to it? I have a feeling that pi and other long forgotten things may come into it!
Thank you for any answers.
David.

[Peter Naulls]

Hi David,

Yes you're quite right, 75mph. 
Pi is just a ratio between the diameter and the circumference so you don't need to involve that.

Peter.

[JonE]

There is a reference table in the Williams book for different tyre sizes and diffs.
It will change the gearing slightly, so if you are underpowered there is greater chance that you won't actually get to your theoretical maximum?

[Alan]

Yes, but be careful in that tyres of same nominal size but different brand might not have the same actual diameter.

[Chris KC]

You need to use tyre dynamic radius rather than static if 'accurate' results are sought.