To save gas and money, an economy car may be the best option for renting, especially if you are going to be doing a lot of driving. Most companies now offer unlimited mileage, but you still have to pay for gas. Keep in mind the bigger the car the lower the miles per gallon the vehicle will usually get.
If you are booking a car in a country overseas or outside of your local area, be sure to check on the type of transmission you will be driving. Many European and South American countries have standard transmissions, which can be a real problem if you are used to a manual transmission.