A 261 kW (350 hp) spark-ignited, turbo-charged and aftercooled Cummins “Big Cam” propane engine was developed and tested for class 8 truck application. Two propane engines were operated in line-haul service for a total of 33 months and almost 350 000 kilometres (217 480 miles). A diesel-powered tractor, in similar service, was monitored for comparison.Fuel economy, oil consumption, maintenance, drivers' comments, and engine refinements were fully documented. Propane fuel economy averaged 94 L/100 km (2.5 mpg), relative to 45 L/100 km (5.2 mpg) with the diesel. Overall reliability was good and driver acceptance excellent; however, further refinements, especially in the turbocharger, ignition, and cooling system, are required to compete with the modern diesel.