Many new riders who just bought a mountain bike don’t know the difference between 21-speed, 24-speed, and 27-speed. Or just know that 21-speed is 3X7, 24-speed is 3X8, and 27-speed is 3X9. Also someone asked if a 24-speed mountain bike is faster than a 27-speed one. In fact, the speed ratio just gives the riders more opportunities to choose. The speed depends on the rider’s leg strength, endurance and skills. As long as you have great strength, a 21-speed one is not slower than a 24-speed bike! How many miles can a mountain bike ride?
In theory, at the same pedaling cadence, a 27-speed bike will run faster than a 24-speed one. But in fact, with a high gear ratio, the pedaling will be very heavy, and the cadence will naturally decrease. If the cadence decreases, the speed will naturally decrease. Sometimes some beginners buy mountain bikes and say, “My bike is good, why is it so hard to pedal?”The reason is that he didn’t choose a gear ratio that suits his when riding.

First let’s take a look at the difference between 21-speed, 24-speed and 27-speed:

21-speed chainwheel & crank 48-38-28 flywheel 14~ 28

24-speed chainwheel & crank  42-32-22 flywheel 11~ 30(11~ 32)

27-speed chainwheel & crank   44-32-22 flywheel 11~ 30(11~ 32)

The gear ratio is the number of gears divided by the number of flywheels

21-speed maximum gear ratio 3.43, minimum gear ratio 1

24-speed maximum gear ratio 3.82, minimum gear ratio 0.73 (0.69)

27-speed maximum gear ratio 4, minimum gear ratio 0.73 (0.69)

From this we can see the difference between them. The 27-speed and 24-speed have a larger or smaller gear ratio than the 21-speed, which can make you ride faster and make you ride less effort. Because the 24-speed chainwheel is not the same as the 21-speed one, the smaller chainwheel can get a lighter gear ratio, which is a big advantage when climbing. A 24-speed bike can achieve a transmission ratio of 1.07 even if it uses a 2X1 speed ratio. If the flywheel is 11~32, it can achieve a transmission ratio of 1 (the minimum transmission ratio of a 21-speed is 1).  So the advantage over a 21-speed bike of a 24-speed one is not only in the fastest gear, but more in the slowest gear, which making it easier and more powerful for you to ride on mountain roads. A new rider only thinks that the 24-speed bike is faster than the 21-speed bike. Maybe few people divide the number of teeth of each crank and cassette to see what is the difference.

As for the 27-speed mountain bike, its flywheel is generally the same as the 24-speed one. The difference is that the largest front crank is adjusted from 42 to 44, which is suitable for people with good physical strength. A 24-speed mountain bike or a 27-speed mountain bike is the difference between the various parts of the bike that have been upgraded to better models with its grade.