While looking for tax rebates offered for purchases of hybrid, plug in-hybrid and other energy efficient vehicles, i discovered fueleconomy.gov from U.S department of Energy.
Besides tax rebate issues, the site allows you prior to purchasing a vehicle to compare up to 3 different cars side by side on fuel efficiency.
Going on a summer holiday trip, you can see your own vehicle fuel efficiency as well as how many gallons you will use and what the cost will be thanks to My Trip Calculator option of the site which also gives you distance involved, time it will take and driving directions.
I tried it with Verona (NJ) as departure and Washington D.C as destination.
Results: My small Nissan Versa is listed at 29 MPG which means that I would use 7.9 Gallons of Regular Gas at estimated $3.72 per gallon for a final cost (each way) of $29.53.
Distance of 230.2 miles would be covered in 3 hours and 42 minutes.
If you have 2 (or more) vehicles in your home, it might be worth checking them side by side as this option is offered and you will know what's best ride.
New Energy Efficiency Labels as one above coming to dealers lots with 2013 models should also help us while car shopping.
Life's a gas, could be an hybrid for Green Day # 236
(* Image of Fuel Efficiency Label from epa.gov website)