"Here I am trying to visualize the gear required to go light speed.
First, I think even the most hard-headed retro-grouches will admit that an 11 cog is appropriate here. For a decent 120 rpm spin, and for a typical 2.1 meter rolling circumference :
front chainring = (3x10^8 / 2.1) * 11 / 120 = 13 million teeth.
The chainring will have a diameter, therefore, assuming a tooth pitch of 0.5 inches, of 53 kilometers."
--Dan Connelly