A pulley could be made out of a car's tire rim without the tire. The multi-rigged wide
V-Belt (typical of today's alternators) rides in the low part making a very large pulley.
Another possibility, a pulley could be made out of 3 round pieces of 1/2" to 1"+ plywood
sandwiched together. This would depend on belt width. The center circular piece would
be about 1" smaller in diameter. The inner edges of the two outer plates would be
rounded or beveled on the inside so as to not excessively wear out the belt. In this case
the pulley could be built as large or small as needed. Rope or string could be wound
around the alternator's pulley to make it even or higher than the diameter of the sides of
the typical alternator pulley. This could be finished off by soaking it with epoxy and
letting it harden. The built-up area would keep the pulley edges from cutting into the car
tire, making the tire last longer. The 2"x4" could be a 4"x4", 2"x6" or 2"x12" as needed.

A typical 14" car tire has a diameter of about 23" to 26" depending on type. Assume 2000
RPM is a good working speed to get useful wattage out of a car alternator. Assume 300
RPM is the maximum speed for the tire-propeller. The alternators that I have measured
range from 2.2" to 2.8" in diameter. Note that:

PM(gen.) times diameter(gen.) = RPM(Tire) times diameter(tire)

If this is solved for propeller or tire speed we get the range of

RPM(tire) = (2.8"*2000RPM)/23" = 243 RPM or (2.2"*2000RPM)/26" = 169 RPM

This indicates that the optimum generator speed is within the range of possibility for this design. Thus it should be possible to efficiently use a car generator for such purposes. The larger the tire and the smaller the pulley, the lower the speed of the propeller for sufficient current to be generated. The optimum propeller design and size could be arrived at by trial and error.

Offered by Mike.