It is in fact true that driving at slightly (key word) higher rpms is much better (for both fuel efficiency and engine life. The rpm range for the two can be different).
Part 1. Engine life:
Every engine has a least resonant rpm. This is when your engine vibrates the least (Think of this as the opposite of that point when your entire car begins to shake violently due to excess load). Since the vibration is minimum at this rpm, the wear and tear in the engine is minimized and so is the friction between engine components. This naturally means you have parts that run smoother and longer. This can also increase your fuel efficiency since frictional power loss is minimized.
Part 2.Optimal fuel efficiency:
This rpm range need not be the same as that for optimal engine life. Your fuel efficiency is best when you are not over working the car. This has multiple dimensions to it. Ideally, driving the car at a high gear, just shy of the midway mark on your tachometer is when the fuel efficiency is the maximum (i.e. when you are cruising) but if you demand acceleration you will burn a lot of fuel to get very little. This type of driving is best for the highway. In the city limits, it would be best to raise the engine to the sweet-spot (where the resonance is the least) and then shift to drop the rpm back down, all the while not being aggressive with the pedal.
Low rpms give you better efficiency for two reasons,
- Dynamic friction is proportional to speed. (Higher the speed, greater the friction). This applies everywhere, from the crank shaft to the piston-cylinder to the gearbox. A car can rev only so much because after a certain point, the power generated is less than that needed to overcome the frictional force.
Bottom line: Lower the speed, better the efficiency.
- Volumetric efficiency. (Higher the speed, lower the volumetric efficiency) At low engine speeds, as the engine goes through the exhaust stroke and the suction stroke, air can easily exit and fill the engine cylinder. As the engine rpm increase it becomes harder and harder for the burnt air fuel mixture to exit the cylinder (in the exhaust stroke) and for the fresh air to enter the cylinder (in the suction stroke). This translates to wastage of power because the engine has to use some of the power generated to push the air out faster and faster. On the suction stroke, the air does not travel fast enough to fill the cylinder in the short duration that the intake valve is open.
(To make matters worse, the ECU measures the pressure at the inlet and expects a certain amount of air to enter the engine and adjusts the fuel injected accordingly, but when the expected amount of air does not enter the engine, the fuel that would otherwise have been burnt gets wasted.)
Bottom Line: Lower the speed, better the efficiency.
Now, remember the least resonance rpm? Remember how coming closer and closer to that rpm, reduces frictional loses? This is around half way through your rev range. Hence you will get the best fuel efficiency when you are in the proximity of least resonance but low enough to take advantage of points 1 and 2.
For easier understanding, look at this graph:
The above graph is for an engine on a test bed. For an engine on a car, the lowest point is a little to the left at around 2500 rpm (just like you stated).
Screw the explanation, give me the answer:
For best fuel efficiency:
When driving on the highway, drive just shy of the midway mark. In the city, rev all the way up to the sweet-spot and then shift up to comeback to just shy of the mid way mark.
For best engine life:
Do what you must to stay at the sweet spot.
How to find the sweet spot:
1.Park the car (handbrake is engaged)
2.Start the engine, put it in second or third gear.
3.While keeping the clutch depressed, raise the engine in steps of about 250rpm (Smaller steps if you are really particular).
4.At a very exact rpm, the car will feel like it is running smoother than usual.
5.Congratulations. You have found the "sweet-spot".
(PS. This is harder than it sounds, but I encourage you to do it!)
(Disclaimer: Sweet-spot is not a universally understood term,
when searching online or talking to your car buddies, use the term
"Point of least resonance".)