You need to know the actual fuel consumption for the specific vehicle you have in mind in order to understand it's true operating costs. It is easy to understand that the bigger the vehicle, the more expensive it is to drive, however even within vehicle classes, the difference in fuel use can be more than double! 

If you are buying a new car you can find the fuel efficiency rating on the car's EnerGuide label.  Its a great comparison tool to help you recognize significant savings.

If you are buying a used vehicle, you can visit: http://oee.nrcan.gc.ca/transportation/tools/fuelratings/ and look up the actual fuel consumption rating for the make and model year of the vehicle.