Timing belts/chains, belts cannot be easily extended or shortened to accommodate different sizes of pulleys. Therefore, it is important to make a good choice in the size and length of the pulley to achieve the desired gear ratio while maintaining the proper center distance. Below is a calculator applet that calculates the dimensions of the front and rear pulleys or sprocket and the length of the belt or chain and calculates the final gear ratio and the center distance (approximation).

Gear ratio and center to center distance

<img src="http://drr.ikcest.org/static/upload/b7/b70e8472-ebd5-11e9-a882-00163e0618d6_m.jpg" />