To calculate your gear inches:

1) Teeth on chainring, divided by teeth on sprocket.

2) Multiply the result by the diameter of the wheel, including tyre.

Example 1: 49 tooth chainring, 18 tooth sprocket, 700c wheel (roughly 28 inch diameter).

1) 49 divided by 18 is 2.72

2) 2.72 x 28 = 76.2 gear inches.

This means that the combined effect of the gearing and wheel size is the same as if you were directly pedalling a 76 inch wheel (a very big penny farthing or unicycle!)

Example 2: 43 tooth chainring, 17 tooth sprocket, 700c wheel (28 inch diameter)

1) 43/17= 2.53

2) 2.53 x 28 = 70.8 gear inches.

This means that the combined effect of the gearing and wheel size is the same as if you were directly pedalling a 76 inch wheel.

You need to factor in the wheel size because a 43/17 set up on a small wheel Moulton, for example, would be a very low gear. More sensibly, in these days of 700c and 650b wheels, and 26" and 29" MTB wheels, you can't just look at the teeth.