When you buy a new bike or change either the Chain Rings or Cogs on your bike you are potentially changing your riding power. 

In order to determine the effect of changes to your Chain Rings or Cogs you will need to make a Gear Chart.  You will need to know the size of your wheels, the size of your Chain Rings (number of teeth) and the size of your Cogs (number of teeth).

The size of the Chain Ring may be stamped on the Chain Ring.  If not, you will need to count the number of teeth on each of your Chain Rings.  You will also need to count the number of teeth on each of your Cogs.  Write these numbers down on a piece of paper.  Write the Chain Rings across the top of the paper and the Cogs down the side of the paper

Find your wheel size – most bikes are either 27” (700c) or 26” (650).  The wheel size will be on the side wall of your tire.

Cogs	Chain Rings
	55	42	34
12	119	91	74
13	110	84	68
14	102	78	63
15	95	73	59
17	84	64	52
19	75	57	47
21	68	52	42
24	60	46	37
27	53	40	33

Now, perform the following calculation for each combination of Chain Ring and Cog.  Fill in the results in each of the boxes.

(Chain Ring x Wheel Size) / Cog

Chain Rings: 55, 42, 34

Cogs: 12, 13, 14, 15, 17, 19, 21, 24, 25

Wheel Size: 26 inches

Big Chain Ring (55) and Smallest Cog (12) –

55 x 26 = 1430

1430 / 12 = 119 inches per pedal revolution

Small Chain Ring (34) and Largest Cog (27) –

34 x 26 = 884

884 / 27 = 33 inches per pedal revolution

Find the Cog you ride in the most and find the chain ring you ride in most.  I’ve indicated that as:

57

Now, repeat the steps for your new Chain Rings and/or Cogs.  Find the gears you ride in most frequently on your new chart.  Also, compare the biggest gear (hardest) and the smallest gear (easiest).  If the smallest gear is larger than the old one, you will loose climbing ability – the new gearing will be harder to climb with. 

33

If the largest gear is smaller than the old one, you will loose speed – the new gearing will not provide as much speed as your old one.

119

Now you have the means to compare one bike’s gearing to another bike’s gearing.

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