Liquefied gas or propane was first used in 1913, but this cheaper gas alternative is now enjoying renewed interest from light- and medium-duty truck fleets due to rising fuel prices and tax credit availability. The cost of a gallon of propane has remained about 30% to 40% less than a gallon of gasoline over a 15-year average. None of the large manufacturers offers a propane truck direct from the factory, but several large companies specialize in converting engines to run on propane at a conversion cost ranging from $8,000 – $16,000 per vehicle.