The majority of cars in the US (rental cars especially) use unleaded gas/fuel which is labeled as `unleaded gas` or just `gas`. Diesel gas is generally used for trucks or large vehicles. Check with the rental car agency before leaving to know what kind of gas to use in the car you hire. Buying gas in the United States is a fairly simple process, each machine offers a credit card option at the pump, fill up the tank and your card is charged. If you want to pay with cash or have issues with your credit card you must go inside and see the attendant, specify the pump number, and how much you want to pay in cash.