Worm gears are great for gear reduction, however they aren't very efficient due to friction. One other pitfall or perk depending on your requirements - once you stop giving your drive shaft power, your axle/tires are LOCKED up. It creates for some jerky movements, but it is nice to have such a drive system if the bot will ever need to 'park' on an incline or decline. This jerkiness can be almost eliminated but ramping up and ramping down power to your motor. Smoothing it out will require a little more coding on your half though.
If I were presented with this issue, I would find the smallest bike sprocket I could find and a much larger sprocket and use a bike chain drive. I'm not sure if that is overkill for your project though or what your monetary restrictions are.