I came across this thread from a random search of the internet. Let me correct a whole bunch of information.
1. As Sick88TBird pointed out, engine speed has to do with gearing and vehicle speed, and that's all. So if a given car goes 2000 RPM at 70 MPH, it will do so as long as it makes enough power at that RPM to overcome friction and drag. Having a lower RPM DOES NOT always realize less fuel being used. If RPM is low enough that you're not making enough road power to maintain your speed, you have to increase throttle input to compensate. So, you could still be at 2000 RPM with a massive head wind, going up hill and be at full throttle, using large amounts of fuel, but not making a heck of a lot of power. This is what's called being "under load."
2. Optimum Air/Fuel ratios change based on whatever condition you're hoping to be in. The optimum air/fuel ratio for best power, least emission and least fuel are all different. Stoichiometric A/F ratio is 14.7:1, and this is the ratio in which the engine burns fuel the cleanest, given fewest emissions. Best power is usually between 12:1 and 13:1 range. Best fuel economy is usually up around 16:1 or higher.
3. Mr. Watt's horse didn't do 33,000 ft/lb per minute, he did 22,000 ft/lbs, but he added 50 % "for good measure."
4. I forget whom said it, but they were correct in saying it requires a specific amount of horsepower to maintain a specific speed. The key is something called Brake Specific Fue Consumption. It's the ratio in which an engine makes the most power with the least fuel being used. It's always highest at wide open throttle, and always before peak torque. I can guarantee that if your car revs at 2500 RPM at 65 mph, then that's probably where your engine is making the most power for that throttle input, and using the least fuel. Auto manufacturers know their stuff, so they design these things that way.
5. A car may or may not get better fuel economy if it makes more power. If an engine makes more horsepower because it uses less friction or has to overcome less pumping losses, then it will be more efficient. For instance, every engine uses up power to overcome friction and pumping losses(energy used to suck in air and blow it out the tail pipe). Let's say an engine has a "rated output" of 200 hp. That engine is probably creating 250 hp, 50 of which are lost to friction (seen as heat) and sucking air in through the intake tract and blowing it out the tailpipe. If that manufacturer redesigns their piston skirts and uses a different cylinder wall material to cut friction, and the engine is now rated at 205 horsepower, then that engine will get better fuel economy. Because if it takes 20 horsepower to maintain 60 mph, then .5 % of the energy that was originally going to overcoming friction, can now go to moving the car down the road.
6. I don't know where the 5252 constant comes from, that's why I did a Google search and found this thread. I'll let you all know when I find it.
1993 Mazda Protege GT - Highly modified
2006 Mazda6 Sportwagon GT - Stock