Converting tire length into mm?

[HillSlugger]

I wrapped a string around my tire (700x32c) and then got the length from a tape measure. On my first ride my distances were all about 5% higher than what my car odometer had said. I decreased the setting by 5% and ended up short on my next ride. I ended up nearly perfect with a -2.25% adjustment. I don't know why this is necessary. Who can be sure that the car odometer is right either?

[DebW]

I wrapped a string around my tire (700x32c) and then got the length from a tape measure. On my first ride my distances were all about 5% higher than what my car odometer had said. I decreased the setting by 5% and ended up short on my next ride. I ended up nearly perfect with a -2.25% adjustment. I don't know why this is necessary. Who can be sure that the car odometer is right either?

The tire circumference is probably different with a rider on the bike. You should have your weight on the bike when you do a roll-out. That probably accounts for your difference. Of course, the car odo may not be that accurate either.