Much like putting aftermarket wheels on an actual car, when making vehicle models, it often becomes tedious trying to find that perfect wheel/tire combination to go with your cool new creation. Sariel has updated his Lego wheel and tire guide to show all the various wheels and tires made by the Lego, the largest tire manufacturer on the planet.
The guide is neatly laid out to show all of the available wheels and their various tire combinations, listing total diameter in metric, standard and Lego measurements. He also includes tire width, wheel width and weight. If you roll over each wheel, a graph populates that shows what tire fits what wheel and vice versa. There are even links on each element over to Bricklink where you can purchase elements on the grey market.