Why can't the gear ratio be something like 100:1? Is it practically possible? I am planning to develop a bicycle on my own for a competition. But the main requirement of it is that it must be able to develop a maximum speed. If I am able to cast the sprockets with gear ratio say 10:1 or something like 100:1, will there be any practical hindrance to it?
The main hindrance will be actually getting going from a standstill.
This kind of thing has been done quite a bit and is called motor-pacing and involves riding a very highly geared (usually fixed-gear) bike behind a fast car with some kind of fairing to reduce wind resistance on the bike. Speeds of well over 100mph have been reached.

This kind of bike is totally unsuitable for riding anywhere but a specially controlled long straight under the right conditions.

Check out this video for an idea of what's involved.