This is a copy-paste from a previous post I wrote. The discussion was primarily oriented to the most economical way to drive, but it also ties in well with optimal RPM for prolonging engine life.
The optimal RPM for fuel economy is entirely load dependent. The engine itself is generally most thermodynamically efficient when at full throttle at around the peak torque RPM. At this RPM and throttle position, the engine is most capable of turning the chemical energy of gasoline into usable work. I don’t know the thermodynamic efficiency of the 4.0, but 40% is a good estimate for many engines in this condition.
However, we rarely need that much power or torque continuously, so at lower power/torque demands, different RPMs are appropriate. At lower torque demands, thermodynamic efficiency drops, and the ideal RPM for that demand changes with it.
Note that “torque demand” and “engine torque” generally do not mean the same thing. Torque demand is torque desired by the user at the wheels, which is proportional to the engine torque times any gear reduction ratios divided by any loss ratios. Engine torque is the torque at the crankshaft only, and is not affected by change in gear ratios (e.g., shifting gears).
At freeway speeds (~70-80 mph), the torque demand is often very high, and an RPM closer to peak torque such as between 2500-3200 RPM is generally appropriate for fuel economy.
At lower highway speeds (~55 mph) torque demand is lower, and you can get good economy at a lower RPM, usually 2000-2400 depending upon actual load.
At very low speeds or situations with very low torque demands, much lower RPMs can be appropriate. If you only need a very small amount of torque to sustain speed down a big hill, RPMs as low as 12-1500 could even be appropriate.
As a general rule of thumb, I like to think of throttle position versus RPM range.
TJs idle about 700 RPM and experience peak torque right around 3200 RPM. If zero torque (closed throttle) is demanded, the most efficient position is idle. If maximum torque is demanded (wide open throttle) the most efficient position is the peak engine torque RPM, or about 3200 RPM.
You can interpolate any of those values using throttle position versus relative RPM. So if I’m holding a position of about 50% throttle, about 2000 RPM is a good target for economy. 75% throttle, and I would probably want about 2600 RPM.
There isn’t an exact correlation between throttle position and demanded torque, but it is close enough that throttle position can be used as a guide for demanded torque. In this case, throttle position is determined by how far down in the travel range the gas pedal is. 50% throttle means the gas pedal is depressed halfway.
Above the peak torque RPM, thermodynamic efficiency tends to drop, however, sometimes the additional power is required to get torque at the wheels. Thus, excursions over 3200 RPM will likely negatively impact fuel economy, but may still be required to deliver the demanded torque at the wheels. Brief excursions should not have a very large impact upon fuel economy, though thermodynamic efficiency (and subsequently fuel economy) begins to drop drastically above the peak horsepower RPM (4600 rpm).
