The engine still runs on diesel fuel. A small amount of propane is injected into the system by a computer controlled device to help the diesel fuel burn more effeciently. It is NOT a retrofit to run on propane.

Regarding the tax, the propane tank is removable and refilled like a gas grill tank. Uncle Sam doesn't tax this the same way as if it was permanently mounted as a fuel tank...yet. The tank on my truck holds only 20lbs, which lasts about 1200 miles.