This is tangential to the original poster's question, but relevant. Bikers have historically talked about gear ratios, but that's only part of the equation when you're considering how "hard" or "easy" a bike is to pedal. There has been a move by some (reference given to Sheldon Brown's website above) to talk instead about Gain Ratios, instead of gear ratios. Gain Ratio is a preferred measure in my view because it is a (unitless) number that takes wheel size AND crank length into consideration, as well as gear ratio. I wish use of this measure would catch on faster. With skinny road bike tires, it didn't make as much of a difference because all the wheel diameters were very close and varying crankarm lengths was not really common. So many old school roadies just talked gear ratios. (I'm kinda guessing this is how it happened, but bear with me.) Now there are a lot more common wheel sizes (20, 24, 26, 650B, 29er), and tire width and effective wheel diameters can be all over the map. (E.g., the Surly Krampus.)
Here's my explanation of the Gain Ratio (just skip to the Bold if you're bored):
Gain Ratio = Bike Travel / Pedal Travel (think arc lengths, s=r*θ)
= radius_wheel*θ_wheel / radius_crank*θ_crank.
Since the angles through which the crank and wheel rotate are related by the gear ratio (number of teeth), substitute using
θ_wheel = (front chainring teeth/rear cog teeth)*θ_crank.
So,
Gain Ratio = (radius_wheel/ radius_crank)*(front chainring teeth/rear cog teeth), or
Gain Ratio= Radius_ratio * (Teeth_chainring/Teeth_rearCog)
This is a unitless number that takes crankarm length AND tire size into account.
If you do a couple of calculations for different bikes, especially stocks vs mods (use data from Inspired's website, for instance), you may gain some insight into why certain gear ratios /gain ratios are preferred, which are equivalent, and how much the gain ratio can change when you make seemingly minor gear or cranklength changes, or when you put on a really fat rear tire.
NB: You should try to use the effective radius of the installed tire in these calcs; measure vertically from ground to center of rear axle.