HiTempguy wrote:
You are wrong. RPM kills mpg, end of discussion. There is so much fuel required to be burnt with a given volume of air. Lower rpm will almost always return better mpg, including under heavy load unless the manufacturer has DESIGNED IT NOT TO (which could be for warranty/heat concerns, I agree with you in that regard). It's pretty simple marth. The higher the rpm, the more volume of fuel/air you put through the motor. I can guarantee you at WOT at 1800rpm is less fuel than 2500rpm at part throttle in almost any situation, again, unless the manufacturer has specifically gone out of their way to do so (warranty and/or poor design).
Also, being "gentle" on a motor is silly. These are mechanical parts, not humans. They only care about operating within their design specifications, anything else you attribute to being "gentle" is all in your head. I've put on over 250k kms towing in my short life, I've never seen these "heat management" problems people seem to imagine.
In theory, you're right. If the motor ran at the same AFR all the time, then less RPM / more throttle would equal better MPG for the same power output.
But in reality, pumping losses aren't the only thing that have an effect here. Best BSFC is found at a fairly lean AFR (lean of stoich). But we can't run a motor that lean under full load, as it'll never keep the combustion chambers cool enough, etc. and we'll detonate it into oblivion (or pull a ton of timing, etc. to make it live and massively compromise power output in the process, at which point we'd have to drop a gear just to have enough power).
Because of this issue, BSFC improves at higher engine loadings, but only up to a point. That's usually in the ballpark of 80% load for most N/A engines. Beyond this, the AFRs have to be richened up significantly to keep the engine alive and allow decent power production. When this happens, BSFC starts to take a nose-dive, as you're moving from the slightly richer than peak efficiency burn you've got at cruise (where it's probably running around 14.7:1 AFR) to a much richer than peak efficiency, but better for power production and cylinder cooling, burn in the 12.5 - 13:1 AFR range.
Because of that, there's a crossover point somewhat short of 100% load where dropping a gear and going from, say, 100% load to 60% load ends up leaving you in a similar or slightly better spot on the BSFC curve and with more reserve power available without needing a downshift.
I've done some testing of this in my Jeep. Climbing a specific hill near home at 40 mph, I have the choice of being in 4th gear or 3rd gear to climb the hill at a steady speed. 4th puts me at ~1200 RPM and has the MAP sensor indicating that I'm right around 95% load (manifold pressure is 95% of outside atmospheric pressure). Doing this, it's running 12.8 - 13:1 AFR the whole way up the hill. 3rd brings it up to just over 1700 rpm, but now the load is lighter (can't remember the percentage off-hand) and it stays in closed loop, which I have set up for a bit of a lean burn. So it's now climbing the hill running 15.5 - 15.6:1 AFR, which both according to the on-board MPG display and a data log of RPM vs injector pulsewidth, has it burning fuel at almost exactly the same rate in either gear.
As you get higher up in the rpm range, pumping losses become a more significant concern, so you will eventually hit a point where dropping a gear will always burn more fuel (such as going 80 mph, where going from 4th to 3rd would be going from ~2400 rpm to almost 3500 rpm). But at the lower end of the rpm range, it's not uncommon for other factors to lead to very similar fuel consumption in a lower gear despite the increased pumping losses.
The heat issue is mostly this: if you're climbing a mountain on a hot day with the A/C cranked, heavily loaded and the vehicle has a mechanically driven cooling fan, you'll have much more cooling airflow to work with if needed when the engine is at higher rpm. For electric-fan vehicles, it's less of an issue, provided the water pump flows enough at lower rpm.
We'll put it this way: if lower rpm always meant better MPG, you better believe every car manufacturer out there would gear their cars to turn 1000 rpm at 70 mph so they'd do great on highway mpg tests and just downshift whenever you asked for any acceleration or hit a hill. But they don't, because past a point, the gains are so small that the loss of responsiveness isn't worth it.
