Fill the tank. Run the engine for 10 hours then fill tank again noting how much fuel you added. Divide that amount by ten and you have your average fuel consumption per hour. You probably have about a 19 gallon tank and either a 15 or 18 hp diesel. So at wide open throttle you would burn about 1 gallon per hour. At lower speeds your fuel burn is probably more like 0.5 gph or less. So if you log engine hours, you can probably add a five gallon can of diesel every 10 hours or so at the high end.