I agree with fredg2013, however it depends. You will have to do the math to find out based on where and how far your going. Diesel gets better gas mileage than a lot of petrol cars, however if you get an electric or a hybrid such as the Chevrolet Volt (I highly reccommend) you'll likely only need one tank of gas to get from Washington to North Carolina. Diesel is more expensive than petrol as well but in some places it is only a little more expensive. It could be cheaper to do Diesel if you are going to/passing through places with cheaper Diesel and you have worse gas mileage in your petrol vehicle.