Use the GPM calculator to compare cars on gas and cost savings:
These power point slides show you how gas consumption varies with a car's MPG and the distance driven. If you own a 20 MPG car, how much will you save by a) switching to a 25 MPG car vs. b) driving 3,000 miles less per year?
These tables translate MPG to gas consumption:
Here is a great calculator from Climate Central that automatically updates gas prices