The savings go beyond the lower cost of the fuel. LPG burns so clean that operators find they can stretch out their oil changes to every 7,000 miles, versus the 4,000 mile oil changes they need with diesel engines.

Better still, an LPG engine only needs 6 quarts of oil in the crankcase versus 16 quarts in a diesel engine. Over the six years or so that a fleet typically keeps a truck or bus, this adds up to a substantial savings in maintenance costs. And then there's the environmental benefit.

"We spent over a year investigating all the different alternative fuels in the marketplace. Propane autogas came right to the top," says Abe Stephenson