I admire Sheldon Brown as one of the wisest and most decent men I've never met.
I only know him through his writing, and, without him, I never would have discovered fixed gear bikes.

That said, I think the following site with its gear inch, gain, development and skid patch applet will explain gear ratios to the average person with fewer burned brain cells than trying to read Sheldon Brown's excellent explanation of these factors.

http://software.bareknucklebrigade.c...it.applet.html

I have ridden every possible chain and cog combination from a high of 82 gear inches to a low of 54 gear inches.

I find 72 gear inches the highest gear ratio at which I can ride without a brake, and this tempts fate, because I can barely do it; but then, I neither skip nor skid.

Presently, because of a six-month medical treatment, I ride at 58 gear inches and love it.

My spin has improved amazingly.
I get around town as fast as ever, and I can brake easily, even on a steep hill, without skipping or skidding.

By the way, I get 58 gear inches with a 42t ring, a 19t cog, and 700X23 tires.

I easily cruise at 15mph at a little less than 90rpm.

Yes, a lot of people pass me going downhill, but I don't even try to go fast downhill these days, and surprisingly, I go faster than before.
I've totally lost the bounce, and only my stamina limits my downhill speed: it takes a lot of energy to keep up with the pedals, and less energy to slow down and descend under control.

I'd like to eventually go back up to 72 gear inches and try to ride without a brake, but I think I'll put on a brake, anyway.