well it's teasing my brain…How fast am I going if I pedal my fixed gear commuter at 80rpm and 100rpm; 27" wheels, 46×17. thanks if you can help

http://www.sheldonbrown.com/gears/

That's quite neat.
Also found this: http://software.bareknucklebrigade.com/rabbit.applet.html from which, I was able to calculate an approximate RPM of 150-160 (!) achieved during the sprint at the end of last nights ride

(gear ratio)*(wheel dia)*PI*RPM*60/(inches in a mile)

so, at 80 RPM; (46/17*27*PI*80*60)/63360 = 17.2 mph

at 100 RPM; 17.2 *100/80 = 21.5 mph

I believe

fbf – you forgot to take into account the tyres. Need to measure the actual diameter inc tyres – so a little bit faster than your calcs

bugger, you're right

Also, I only took PI to 2 decimal places

fek, I'm going to the special hell reserved for over assuming mathematicians 😆

Depends how fast the conveyor's going.