Twenty years ago top-level bikes had 10 or 12 speeds while the rest of us made do with fewer.
Now you can buy a bike with 14 or 16 in any bike shop, and the professionals are using 18 or 20.
You might be asking yourself why this is necessary when a car - which travels much quicker - manages with just five or six gears.
A family car's engine will turn at up to 6,000 revolutions per minute while even the best cyclists' legs won't do a "cadence" of more than 130rpm.
That means different gear ratios must be much closer together, and since riders prefer to keep as constant a cadence as possible, the more gears the better.
Mechanically the gear systems are similar to those found on a £150 mountain bike, with the derailleur system having been used in the Tour since 1937.
As ever components are getting lighter, while the method of changing gear has also changed in the past 15 years.
Riders no longer have to reach down to a lever on the frame and can now flick shifters inside their brakes - safer and quicker if slightly heavier.
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