It would make sense to me that in order to get the best gas mileage while cruising, you would want the engine to run at as low of an RPM as you cam. This is where the engine working hard and has been called the "lug zone" by some...

Some people are telling me that they get better gas mileage by watching a vacuume gauge and cruising at the RPM that shows the highest reading. Sometimes this involves running in 5th instead of 6th gear for more RPM.

I'm also wondering if it would be best to cruise when the engine is running at it's peak torque rating which would also be where it's getting the highest volumetric efficiency.

This has been puzzling me for a while.

 

I read many times that an engine needed to be in its "efficient" operating range to get best mileaage. This seems to be an old wives tale as current thinking is generally that keeping the rpm the lowest without bogging or lugging the engine will generally produce the best mileage. This may not be true if using a wild cam that is far below its intended operating range (not generally an issue unless cam duration exceeds the 230's) but then you cannot drive it at low rpm anyway due to surging etc.

I have never increased the rear gear ratio (numerically) & picked up mileage on a fuel injected car. I have on a carbureted car, but I will blame this on carb calibration as this is generally far from optimum.

Remember if a car only needs 20hp to cruise down the road, there is no need to operate it where it can make 200.

Keeping the engine at maximum vacuum as you stated should give the best mileage as this is where it is consuming the least air in relation to a specific rpm.

Putting things in perspective 30" vacuum is -1 atmosphere. This hypothetically means an engine is flowing 1/2 of its rated flow. If you could operate an engine at 4000@ 30" it should consume the same air mass at 0"@2000, but do not expect the same mileage.

As you increase rpm you generate more friction in the engine.
Increased vacuum also makes the engine work harder as the pistons have to pull harder on the intake charge to get it in the cylinder. This all negates some of the high rpm high vacuum theory of mileage.

Keep in mind this is not rigid fact & your results may differ.

Hope this helped.

 

Lonnie said it well.

Part throttle cruising @ 20-50 hp (higher speeds) is completely different from WOT full power conditions. As he said, fricton losses are important when the friction hp (which never gets to the flywheel) is about tha same as the flywheel hp needed to cruise the car as it would be when buzzing the engine.

On a typical 5 speed car, cruising in 3rd rather than 5th can cut the fuel economy in half. I've tested that on a car with a mileage computer: 'tis true.

For a V8 Camaro or similar with a stock or nearly stock cam, keep the engine revs as low as possible above say 1000 rpm. An stock LS1 or LS6 cruuises very well at 1200 in 6th. The LS6 (in a Z06) easily keeps up with most traffic at that rpm without lugging or using much throttle. Remember when OEMs full throttle dyno test, they start as low as the engine will run. Volumetric efficiency at 1200 WOT isn't very good, so the loads on the engine are not high. If you have to use WOT to keep up with traffic, down shift a gear or 2 or 3 (for very fast traffic!).

Maximum mpg also depends on the vehicle drag. I do know that my low drag C5 automatic with 3.15 gears doesn't get quite as good economy as a 2000 SS Camaro with M6 on a 70-80 mph interstate trip. We ran in convoy thru full tanks, carefully topping up each time, and the SS got about 1.5 mpg better. Of course it was turning a over 400+ fewer rpm. If both had M6s, the C5 would have been slightly better due to lower aero drag. I have never found a driving situation where my C5 gets better mpg in 3rd (even with TC locked up) than in 4th.

In the old carb days, you could achieve best mileage with a small carb (or on the tiny primaries of a Quadrajet) running with a lot of blade opening (low vacuum), high compression ratio and dual exhausts (for efficiency) and as low a final drive as possible. This was before OD transmissions. 2.41 rear gears were common, and a 2.29 was sometimes used. With a .50 sixth gear and a 3.42, the modern final is 1.71. With a 3.15 in an A4, fourth is 2.21.

There are some cases like small displacement , somewhat underpowered cars, where 55-65 mph cruise might be near torque peak rpm where VE (at least at WOT) usually peaks. Of course that also means the car will accelerated reasonably in traffic around torque peak and lots of throttle.

 

You've got three main factors effecting efficiency, from the standpoint of the engine:

1) Combustion efficiency

2) Internal friction

3) Pumping losses

Maximum combustion efficiency occurs at peak torque for a given manifold air density. People seem to thinking "peak torque" and instantly go searching out the RPM at which peak torque exists at WOT, not realizing that you're not going to reach your torque peak at 4K with a 5% throttle opening

Internal friction is related to revs, of course, for a given set of engine characteristics. Slower is better, with little exception (certain accessories may be more efficient when driven at higher speeds).

Pumping losses are also related to revs when talking about fixed restrictions, like exhaust, but the intake restriction is also highly related to throttle opening. The smaller the throttle opening, the more work that's required to drag the intake air charge through it, and this is why lean-burn motors (and diesels!) provide a fairly significant increase in fuel efficiency.

So the bottom line is that low engine speeds and large throttle openings are typically going to be most-efficient, assuming that you can stay out of the fueling system's power enrichment mode.

You could attempt to model this sort of thing with your own vehicle, but the accuracy is going to greatly depend on your ability to guess certain parameters that will be very difficult to measure yourself. The OEMs use complex computer models to dial-in their powertrain choices as the vehicle design evolves, and have found that they can correlate within a few percent or less to the actual production vehicle.