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Low Gear Ratio

"Low gear" is the ratio of the smallest chainring on the front and the largest cog on the back cassette. If you've got a 24 tooth chainring on the front and a 36 tooth cog on the back, your low gear is 24/36 teeth =.66 to 1. One turn of the crank turns the wheel .66 of a revolution.

On touring bikes, you definitely want your low ratio to be less than 1 to 1. For example, the Surly Long Haul Trucker comes with a 50/39/30 on the front and a 11-36 on the back. So the lowest gear ratio is 30/36 which is 0.83. My Miyata 1000 has 50/40/28 and a 14-30 on the back, giving 0.93, and my Rocky Mountain is 42/32/22 and 12-32 giving 0.69.

Of course, the gear ratio alone doesn't quite tell the whole story, because it doesn't take into account the diameter of the wheel and tire. There is a measurement called "Gear Inches" which just multiplies the above ratio times the diameter of the tire. eg: A 1.00 cog ratio with a 26 inch wheel would be 26 gear-inches. That is a handy number to compare if you are trying to compare two radically different sizes of wheels, such as a foldup bike and a road bike. If you really want to be fancy, you could factor in the crank length, and make adjustments for the actual tire diameter you have.

As a practical matter, if you are looking at buying a bike, simply calculating the cog ratio is good enough.

  Bike Fraction Ratio WheelD Gear-Inches -----------------------------------------------------
  Surly LHT 30/36 0.83 27" 22.4
  Miyata 1000 28/30 0.93 27" 24.2
  Rocky Mountain 22/32 0.69 26" 17.9

  Sheldon Brown also has a "Gain Ratio" calculator.