Truck engines currently reject up to 40% of the total fuel energy in the exhaust. Because of increasing petroleum costs there is growing interest in techniques that can utilize this waste heat to improve overall system efficiency. This paper examines and compares improvement in fuel economy for a broad spectrum of truck engines and waste heat utilization concepts.The engines considered are the Diesel, spark ignition, gas turbine, and Stirling. Principal emphasis is placed on the turbocharged four-stroke Diesel engine. Because of increased exhaust energy and a large potential improvement in performance, the still-to-be-developed “adiabatic” Diesel is also examined.The waste heat utilization concepts include preheating, regeneration, turbocharging, turbocompounding, and Rankine engine compounding. Predictions are based on fuel-air cycle analyses, computer simulation, and engine test data. All options are compared on the basis of maximum theoretical improvement. The Diesel and adiabatic Diesel are also evaluated in terms of maximumexpected improvement and expected improvement over a driving cycle.The results indicate that Diesels should be turbocharged and aftercooled to the maximum possible level. Based on current design practices fuel economy improvements of up to 6% might be possible. It is also revealed that Rankine engine compounding can provide about three times as much improvement in fuel economy as turbocompounding, but perhaps only the same improvement per dollar. By turbocharging, turbocompounding, and Rankine engine compounding, driving cycle performance could be increased by up to 20% for a Diesel and by up to 40% for an adiabatic Diesel. The study also indicates that Rankine engine compounding can provide significant fuel economy improvement for gas turbine and spark ignition engines and regeneration could significantly enhance the performance of spark ignition engines. Because of the low heat content in the exhaust of a Stirling engine it has only a small potential for further waste heat recovery.