[76GMC1500]

I used to subscribe to the bigger motor may get better milage because it doesn't have to work as hard. Now, I have a better understanding of the dynamics of a running engine. There are 3 major reasons diesel engines get better fuel economy than gas engines. They have higher compression ratios, they have a higher heating value for their fuel (not much higher, though), and they have no throttling losses. Our fuel limits what we can do about the first two, but we can do something about the throttling losses. Open the throttle more. The engine burns more fuel per hour when you open the throttle more, but burns less fuel per horsepower per hour. It takes a certain horsepower to run a certain speed, so the engine with it's throttles open the most is going to be burning the fewest gallons per hour at a given horsepower and will get the best milage as a result. Smaller engnes require the throttles be open more to make the same horsepower as a bigger engine. In cars, bigger engines sometimes have an edge, though. Big engines make a lot of torque at low engine speeds. With big engines in cars, we use overdrive gears to really bring the engine rpm down. You may have heard of the equation horsepower = torquexrpm/5250. Well, in order to maintain speed, we still have to make the same horsepower despite the decreasing rpm. In order to do so, we have to increase the torque output. Torque output is controlled by throttle position. So, with an overdrive, we have to open the throttle more to make a given horsepower, our pumping losses are reduced, and our mpg go up. You might ask why can't I go wide open and run down the highway at 130 mph for best milage, then? Opening the throttle more only improves the gallons/horsepower/hr. At 65 mph, it takes 35-50hp to keep a car moving. At 130 mph, it takes 250-300 hp. To go double the speed, it takes 4-6 times the horsepower. Well, the difference in gallons/hr/hp between an engine with a barely open throttle and a wide open throttle isn't 4-6 times, it's more like 10-20%.