Originally Posted by Silver350
But I can tell you that my big truck will not go up a big hill any measurable amount faster in a lower gear at higher RPM. The difference will be equal to a diff ratio or tire size change.
It will however use more fuel.
Engine size versus design RPM is an interesting subject; although it can get a little complicated. You are correct in implying that engines are more fuel efficient at lower RPM. Typically the sweet spot on efficiency curve is at or below the maximum torque RPM. In your case the 5.4L V8 will use less fuel at 2500 RPM or lower than if you shifted down and climbed a grade in a lower gear at much higher RPM.
However, it gets a little complicated when a driver is after overall fuel economy when choosing an engine size. An example that comes to mind that I've been kicking around is this:
Ford rates the 4.6L V8 in the E-Series at 225 HP at 4800 RPM, and at 286 lb-ft at 3500 RPM. The 5.4L V8 is 255 HP at 4500 RPM and 350 lb-ft at 2500 RPM.
Assuming appropriate gearing is possible, the 4.6L produces more power (190 HP at 3500 RPM) at maximum torque RPM than the 5.4L (167 HP at 2500 RPM) at its maximum torque RPM. Therefore, a 4.6L V8 could do the same job you are doing by simply spinning faster in a lower gear during the hill climb in your example.
I don't doubt that your 5.4L would be more fuel efficient at 2500 RPM than a 4.6L at 3500 RPM, but what happens the rest of the time when cruising on level roads? What's the cost of running an engine that is too large and running it at lower partial load?
While engines run more efficient at lower RPM (maximum torque RPM or just below), the greater factor that affects fuel economy is how much load is on the engine. Gasoline engines need to be loaded at least 50% of available torque before they start to become efficient. By the time they get around 75% of available torque they approach maximum fuel economy.
If a van like yours had a 4.6L versus a 5.4L, it might use more fuel while towing a large trailer, but it's also likely it could save fuel when running without a trailer. So in a case like mine where I rarely pull a trailer, replacing my 6.8L V10 with a 4.6L V8 should save significant fuel most of the time. With the same gearing I could see an improvement of about 17% at slow cruising speeds (below 70 MPH).
Depending on how fast I wanted to drive, I could save even more by going to an even smaller engine. That's why new Transit and ProMaster vans will have V6 engines in the 3.6 to 3.7 liter range. A V10 like mine is very wasteful most of the time -- for my driving conditions anyway.