Here's a video covering the pulley swaps, and custom tensioner pulleys I've built and installed on our off road and mudding tractors. Pulley swaps make you lawn mower faster. You want to increase the size of the front, and/or make the back pulley smaller to increase the speed of your lawn mower or tractor.

Green MTD has a 4" front, stock rear

Craftsman crawler has stock size pulleys

Murray has 4" in the front, and 5" in the rear