Why can't the gear ratio be something like 100:1? Is it practically possible?
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.