Fuel efficiency sometimes means the same as thermal efficiency. This is the efficiency of converting energy contained in a carrier fuel to kinetic energy or work. But fuel efficiency can also mean the output one gets for a unit amount of fuel input such as "miles per gallon" for an automobile. Here, vehicle-miles is the output, but for transportation, output can also be measured in terms of passenger-miles or ton-miles (of freight). While the thermal efficiency of petroleum engines has improved in recent decades, this does not necessarily translate into fuel economy of cars, as people in developed countries tend to buy bigger and heavier cars. Non-transportation applications, such as industry, benefit from increased fuel efficiency, especially fossil fuel power plants or industries dealing with combustion, such as ammonia production during the Haber process.