custom size tire circumference

[jrajr]

I have a new mountain bike! iDash setup doesn't list the tire size but has a custom selection, my tire size is 29x 2.75, so what number do I enter into the custom screen?
Thanks.

[lorduintah]

I believe the answer is 29 * 25.4 * PI or approximately 2314 mm.

This should get you close - Alternative is to mark out a complete revolution of the wheel on a surface and measure that distance the tire rolled.
Tom

[jrajr]

Tom,
Thanks for that calculation. I was wrong on the size though, it is 29x2.1
I would re-calculate myself but am unsure of the 25.4 figure.

Thanks

[lorduintah]

25.4 mm per inch. That's where the conversion comes from.

Tom

[jrajr]

Thanks. I'm American, don't understand the metric system!