Thats intresting. Yes it's true that RPM isn't necessarly related to MPG. An engine should operate within it's comfortable RPM range for best MPG. For example, some engines will run at 4,000 RPM and get what some get at 2,000 RPM. Even though the fuel is injected more frequently at a higher RPM, a less amount of gas is injected due to a lower workload on the engine [as in not lugging it out where theres no power].
BUT, some engines that like low RPMs need to stay at low RPMs for best MPG. If all the powers down low, it's wasteful to rap it out and it works it without no power and more frequent gas injections. But yea depends on the engine.
I personally don't care a lot about HP. Torque is raw force, HP is just the force combined with speed. Think about it like this. A guy doing a bench press. Lets say he can bench 200 pounds. He can push 200 pounds up in say 4 seconds. Now say another guy can do 100 pounds but in 2 seconds. They's exerting the same amount of force essentially.
Or 2 football players. One guy is 300 pounds, one is 150 pounds. The 300 pound guy can run a top speed of 6 MPH. The 150 pound guy can run 12 MPH. If they run they fastest into each other, it's the same force. They would take equal hits force wise. Thats why a little guy can knock a big guy down with the right speed and technique.
Or more relevent to motorcycles. Lets use bicycles. Ok think of 2 guys. A toned, racing bicyclist, and a big strong powerlifter. Both has the same exact bicycle and the same exact gearing. In a race, the pro bicyclist will probly win. The strong guy can probly exert 400-800 pounds to the pedals. He may even win off the line. But the pro will win performance wise for the most part. Now, put a 1 horse trailer with a horse inside behind the bike. I bet the pro will barley be able to pull it, the strong guy much easier.
Also, lest say they bicycles is tied to a wall. The strong guy can stand and put all his force to the pedals, as well as the racer. He is still putting down torque. It don't need any RPM. Torque is raw force. HP is RPMs. The racer may make more HP by pedaling faster even with less force.
Related, but not the same. I have a Dodge Ram 3500 Cummins diesel. A BMW sportscar will smoke it in a race. It has more HP. But lets see it pull 25,000 pounds of round hay bales at 70 MPH up a steep grade! My diesel has a redline of just 3,375 RPM or so. It makes all it's power from only 1,600 RPM and up. It only has 230 or so HP, but it has gobs or torque, something like 460 ft/lbs@1600.
Its not just torque, its RPM. A Harley is pretty slow vs even a small Japanese inline four. But going up a huge steep highway grade, with a big fat passenger, saddle bags and rear trunk full to the max, on a super windy day at 70+ MPH, the torque is what makes it barley even necessary to downshift. That four will struggle and need to get closer to the redline, the H-D will hardly brake a sweat. But in a drag race the Harley is laughable, a Ninja 250 will compete with it, but that Ninja won't run up that hill like that all relaxed at 2,800 RPM.
A H-D will get 46 MPG or so at 2,900 RPM just like a Japanese will get the same at 4,000 RPM. Its all about what its designed to do.