What's the best way to compare gas mileage of cars?

I'm new at car (used) shopping by myself, and I'm trying to research some information. I've been trying to compare gas mileage (among other things) but haven't been able to find anywhere on the web where I can compare several side by side. Can anyone help?|||The site below should get you started on the right track.|||I know for fact that Honda Civic is the best on gas. You'll have to test drive them. Put $5 worth of gas in each car you test drive and see how far this takes you.|||just go to the car dealer ship and they will just give you the answer