Ok, here is the question, and I have never heard it answered before, at least not in a way that satisfies my curiosity.

In a modern gasoline engine, is the fuel economy dictated more by throttle application or by engine RPMs? Basically what I am asking is this, all towing and performance issues aside, if I am driving up a moderate incline with the throttle application remaining unchanged at, say, 3/4 throttle, and the transmission downshifts out of overdrive, is my fuel economy affected? The same applies to driving on flat ground. If I press the O/D off button and the truck picks up speed or maintains speed, is the fuel economy affected simply because the engine is turning more RPMs?

Also, does it matter whether the engine is a new EFI engine or an old carb motor?

I am looking for an answer that is clearly explained and contains more than just anecdotal evidence or personal opinion.

My intent is not to start another endless debate about MPGs or to eek out a few more MPGs out of my truck (if I wanted fuel economy I wouldn't be driving a V8 powered, full sized truck). I am just asking a theorhetical question. TIA --DIY

 

say you were going 70 mph in OD at about 2k rpm (thinks thats about right for a stock f150) and you click out of OD you rpms are gonna jump to nearly 3k and it will drop significantly. the harder you press the accelerator basically dumps more fuel into the engine, the higher RPM's you have the more fuel is needed to keep the engine at those RPMS. thats basically the extent of what i know. not sure how accelerating from a dead stop what is better, going to a slighter higher rpm to get the the speed limit slightly quicker or slowly easing into the peddle to get up to the speed limit. would love to have that clarified for me

 

See though, that's not what I mean. I'm not talking about pressing the accelerator harder. I am talking about pressing it the same, or maybe even less, and the RPMs jumping. I can often kick down out of O/D on a moderate incline and gain speed without pressing any harder on the accelerator. So by definition I can lay out of the throttle and maintain speed. How does this affect things?

 

To simplify greatly, the amount of fuel you consume is largely determined by how much load the engine is under (how much power you need to generate to maintain that speed).

So don't expect huge mpg differences whether your are in OD or not in OD if you are pulling a heavy load and the engine is working hard.

Since it takes some power to make the engine turn more rpms, of course there will be more friction loss at high rpm.

However, most engines are most efficient at or near the rpm where the torque peaks.

When an engine is lugging under heavy load at low rpm and struggling to make enough power to pull that gear, it is generating incredible heat and generally not operating very efficiently, even though rpm is low.

So unloaded, you will get better mpg on the highway at low rpm, low throttle openings, rather than high rpm. Heavily loaded, pick a throttle setting and gear that keep the engine near its peak torque rpm.

 

Ok, but does the additional friction loss at higher RPMs offset the additional power generated? In other words, my question still stands, which uses more fuel? The slightly higher RPMs nearer the peak of the torque curve, or the lower (bogged down) RPM's? The engine load stays the same (I am just maintaining speed). Are you saying it makes no difference? I guess I can see both sides of the coin, the engine pumps more air at higher RPMs (more air = more fuel) but it is much more efficient than a bogged down engine at lower RPMs.

 

Would make a difference how much power the engine is making at the given rpm when bogged down and when revved high. Not just caring about the peak, but how much power is being made at the particular rpm.
A spiky torque curve (Cummins) could greatly change the results from an engine with a flatter torque curve (PSD).
I assume you also mean after engine warm-up where the computer is operating on an open-loop system as well, right?
Given that both are traveling the same amount of distance, I think that the higher-revved engine will use more fuel (if it has an open gear to rev itself in and continues to build speed).
How low the engine is bogged also plays a factor. If it is too low to effectively maintain speed at a lower rpm, then it will use more fuel to travel to same distance as the higher-revved one.
Oh, and do both of them have a standing start when traveling the distance?

 

Consider that the engine is producing enough HP at a given speed to move the vehicle and maintain that speed. Does it effect fuel mileage to drop back a gear and have the rpms higher? Yes, but only marginally. You are having to deal with faster rotating mass and friction in the engine. You also need to consider that at higher gears the engine speed is down and so is the air volume and therefore the gas volume. But there is a point of diminished returns where the engine is being lugged and isn't producing the power that is available in the gas and a lot of it is going out of the tailpipe as pollutants. Likewise, you can run an engine at too much rpms and the engine is working hard just to maintain the elevated rpms causing less fuel mileage. Most of our Tritons work their best at around 1500-2600 rpms and still get decent fuel mileage. Keeping the tach, for normal driving, under 2000 rpms is ideal. But few drive that easy. Unfortunately, I drive like that. You don't want to be behind me because I'm not going to get in any kind of hurry. My tach rarely ever gets over 2000 rpms and I get 17-19 mpg town, 20-24.5 hiway. I have a Supercrew, 4.6, stock.

 

I guess that reinforces what I suspected, but had never heard conclusively. What started the whole thing is that long ago, I had a 1970 F-100 with a 302. I took out the three on the tree and installed a 3 speed with overdrive top loader that I found in a junkyard. I never really saw any improvement in fuel mileage on that thing even after installing the O/D tranny. I ever tried running a whole tank of gas through it without using the O/D and it seemed to get slightly better mileage (without the O/D). It seems to me to be the same deal when you see the same vehicle offered with two different size engines and often in real world driving you find that the mileages between the two are very similar, or in some cases, the mileage for the larger engine is marginally better because it is not laboring as hard to move the same vehicle down the road.

 

It really depends on the type of driving you are doing. If you're in town, you may as well turn the OD off unless you're not in stop and go traffic. Anything under 2000 rpm requires more torque to gain speed. The down shift lets the engine get into its torque curve, and uses less fuel. An example that could show you, would be to cruise control at say 50 mph, on a slight uphill grade.
Turn your OD off and just watch the gas pedal. Once it shifts, the gas pedal will decelerate a small amount. If you are doing 65, it will accelerate. IMO