If you go bigger and heavier, the mileage will be proportionally less. No one expects the price of fuel to be less in the coming years and one of the proposed EPA requirements for lower sulfur in gasoline may increase the cost quite a bit.
If fuel costs are a big concern, you will just have to make up the difference by traveling less, using every discount card that you can find, and adjust your driving habits. Depending on how high the cost of fuel gets, you may have to give up eating while on the road. :D