A diesel motorcycle is a motorcycle with a diesel engine. With a traditionally poor power-to-weight ratio, most diesel engines are generally unsuited for use on motorcycles which normally require low weight, compact size, high RPMs and rapid acceleration. In the 1980s, NATO forces in Europe standardized all their vehicles to operate with diesel fuel. Some forces had fleets of motorcycles and trials were conducted with diesel engines on these. Air-cooled single-cylinder engines built by Lombardini of Italy were used and had some success; achieving similar performance to gas/petrol bikes and fuel economy up to 200 miles per gallon. This led to some countries re-fitting their bikes with diesel engines. In 2005, the United States Marine Corps adopted the M1030M1, an off-road motorcycle based on the Kawasaki KLR650, and retrofitted it with an engine designed to run on diesel or JP8 jet fuel. Since other U.S. tactical vehicles like the HMMWV utility vehicle and M1 Abrams tank also use JP8, adopting a scout motorcycle which runs on the same fuel would ease logistics. Further development by Cranfield University and California-based Hayes Diversified Technologies led to the production of the Kawasaki KLR650 based motorcycle for military use. The engine of this motorcycle is a liquid-cooled, single-cylinder four-stroke which displaces 584 cc and produces 21 kW (28 bhp) with a top speed of 85 mph (136 km/h). Hayes Diversified Technologies mooted, but has subsequently delayed the delivery of a civilian version for approximately US$19,000. In India, motorcycles built by Royal Enfield could be bought with 325 cc single-cylinder diesel engines because diesel fuel was much cheaper than petrol (gasoline) at the time, and of more reliable quality. These engines were noisy and unrefined and not very popular because of lower performance and higher weight penalties. The engines were originally designed for use in commercial applications such as electric generators and water pumps. As of December 2006, several companies produce diesel motorcycles.