Learn something new every day ! ! !
Greenpower racing looks like fun, I had no idea it existed.
Have you heard of
Electrathon racing ?
re : gearing . . . most of the human powered speed bikes will multiply the RPM with a secondary shaft.
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In your case the RPM needs to be divided but the same approach can be used.
Typically the secondary shaft is created from a repurposed bicycle freehub body as shown in photo.
Example shown in speed bike photo is from a BMX source but a road bike derailleur style like shown below might add some gear range.
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The following picture shows an example of a bike with a secondary shaft and dual derailleurs.
The bike is a 35 speed sporting a extra low gear for climbing and extra high gear for speed.
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Is there any efficiency loss with this scheme ?
With only 2 more bearings in the system it will be low.
Also, on the speed racer bike, the bearings are replaced with high quality ceramic bearings with dust shields ( rubber contact seals are removed )