motor/gear ratio

[Billfred]

There are a few ways to make your robot go faster:

-Larger (though not necessarily wider) wheels
-Gearing up, either in gears or (probably easier) sprockets

That said, there are upper limits on that. You're always trading off top speed for acceleration abilities, and demanding too much torque from your motors (read: more than they can put out at stall) is a sure-fire way to trip breakers. The often-used rule of thumb around here is that a robot should be able to slip its wheels when pushing against a wall on carpet. If you want to be able to go faster than that condition allows, you'll need to integrate a multiple-speed transmission.

If you're really up for number-crunching, try JVN's spreadsheet on the topic. Ask around on anything that's fuzzy, and you should get the answers you need.