Bicycle generated power

[Skvez]

Lightspeed

I had been under the impression that a car alternator powered by a bike wasn't very practical because the car alternator needed such a high rpm. However doing a bit of maths suggests it's reasonable:

Typical bike wheel diameter 0.63 m
Typical bike wheel circumference 1.98 m
Reasonable power 'bike speed' 20 mph (miles per hour)
= Reasonable power 'bike speed' 533 mpm (meters per minute)
= wheel rotation of 270 rpm

Given that a car charging alt really needs 2500 to 3000 rpm to get good generation means we need a 10:1 ratio from the bike wheel rpm to the alternator rpm.
If we have a belt on the 0.63m diameter bike wheel we need a 6.3cm diameter pulley wheel on the car alternator.

This seems quite reasonable.