When you buy a TerraPass you're effectively funding research and development in environmentally friendly energy sources roughly in proportion to your vehicle's CO2 output. The idea is to get you to give them money, not to accurately calculate your CO2 output based on your driving style. I'm sure some people actually give more than they need to. Actually, I remember reading on the site that several of the members don't even own cars!

I signed up today. Tom, thanks for pointing this out. I would probably have missed it otherwise.