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For instance, in one book (which was much better than most), the author said that a 27-inch gear was the equivalent of walking.

I'm not sure what 27-inches means. I have a 28 tooth back/42 front as my minimum. One day climbing Centiniel drive - to the Lawrence Hall of Science, I noticed an old guy walking was quite able to keep up with me.

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It's the gear ratio times the nominal diameter of the wheel. Your gear ratio is 1.5:1. Assuming it's a road bike, the nominal diameter is 27 inches. You would have a 40.5 inch bottom gear. This is a rough, easy-to-calculate way of comparing gearing on different bikes, ones that don't necessarily have the same diameter wheels.

There are better methods around, but this is probably the most popular. I imagine it's because the better methods involve harder calculations.

edit: grammar, punctuation.

I'm not sure what 27-inches means

It's the distance the bike moves in one revolution of the crankset

Nope, that would be "27 inches development". "Gear inches" refers to the nominal wheel diameter multiplied by the gear ratio. Approximately, development == gear inches * pi.

I prefer to use development as my unit, taking into account the actual circumference of the wheel and tire. Only at that point can you compare apples-to-apples among a broad range of bicycles.

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