What do Americans call the fuel in a gas-powered car?

There are a variety of different gases that can be used to power vehicles in the U.S., and none is especially common at the consumer level. They are by and large the stuff of commercial and government fleets and a few alternative fuel enthusiasts.

As such, there is no generic term for all vehicles powered by gaseous fuels, and no generic term for all gaseous fuels. You need to specify which gas, and sometimes which form of the gas is in use.

Based on some of the other answers given, perhaps we should start by defining natural gas. In North America, this term applies exclusively to a blend of fuel gases dominated by methane. NaturalGas.org, a site sponsored by the Natural Gas Supply Association (an industry group of producers and distributors), provides this typical composition of refined natural gas:

Methane 70-90%
Ethane, propane, and butane 0-20%
Carbon dioxide 0-8%
Oxygen 0-0.2%
Nitrogen 0-5%
Hydrogen sulphide 0-5%
Rare gases trace

If you were to say you drove a natural gas vehicle (or that your vehicle had a natural gas engine or was natural gas-fueled), the assumption is that your fuel is indeed methane-based natural gas. Biogas (biomethane, renewable natural gas/RNG) refers to the sourcing of the gas as opposed to its composition or properties, so referring to a vehicle as a biogas vehicle is an exercise in ego, as opposed to engineering.

The most commonly distributed format is compressed natural gas, or CNG; liquefied natural gas or LNG is still rare. I have found CNG in consumer magazines, but the average person may or may not understand the distinction between the two, or between them and uncompressed natural gas (as for example delivered to homes for heating and cooking). Those in the transportation or energy industries may be more familiar.

The next most popular alternative vehicle fuel as of 2009 is liquefied petroleum gas, LPG, which is a blend dominated by propane and thus commonly referred to simply as propane. This gas is a common heating, cooking, and lighting fuel in rural areas and in camping equipment, and to the lay person, a propane engine or device is one that runs on LPG, as opposed to propane in the strict sense of the chemical compound, for example in the catchphrase of Hank Hill on the sitcom King of the Hill: I sell propane and propane accessories.

The term autogas for LP gas does not seem to be widespread; note, for example, the absence of autogas engine in Google NGrams against LPG engine and propane engine, which are themselves rare against diesel engine and gasoline engine. The only place it turns up on the U.S. Energy Information Administration website is in a description of natural gas liquids: "a popular fuel in some parts of Europe, Turkey, and Australia."

The fuel for hydrogen engines, whether internal combustion or fuel cell, is simply hydrogen, and more than appropriate as unlike the other gases, it is not a blend of consumables. But hydrogen vehicles are barely more than a laboratory curiosity.

The terms used to refer to LPG are propane and autogas:

• liquefied petroleum gas

• (Elements & Compounds) a mixture of various petroleum gases, esp propane and butane, stored as a liquid under pressure and used as an engine fuel.

(AHD)

Propane vehicles:

• Propane, also known as liquefied petroleum gas (LPG), or autogas, is considered an alternative fuel under the Energy Policy Act of 1992. According to the Propane Education and Research Council, there are more than 143,000 on-road propane vehicles in the United States. Many are used in fleet applications, such as school buses, shuttles, and police vehicles.

(Alternative Fuels Data Centre - US)