Gas mileage

During Thanksgiving weekend my favorite son-in-law (out of a total of ONE SiL) and I were driving to the golf course in the Sierra foothills in my 2006 Subaru Baja. On the downhill sections of the road I put my automatic tranny into a lower gear (this is a recent addition to my driving prowess - at the direction of my favorite son (out of one son), and I complained to my fav SiL that the RPMs increased substantially when I downshifted, and that I was disappointed to be getting lower gas mileage when my foot was off the gas than when I was driving in the normal driving gear. My fav SiL said that was not true, that I was getting better gas mileage despite higher RPMs in the lower gear. Needless to say we couldn’t convince each other of the correctness of our respective positions, so we finally agreed to let you two characters set us straight (though it seems as though you two could use some of the same help yourselves). What say ye? Do I get the bragging rights, or does my soon-to-be less-favored SiL (if you tell us that he is correct)?

Mike Mullery

If you are going the same speed, higher RPMs always means worse gas mileage. The advantage of using the lower gear is that you don’t need to ride the brakes. If the brakes overheat half way down the mountain, you can’t stop. This is more an issue for a large truck rather than a passenger vehicle.

Some newer cars (not sure if your Scooby is one of them) pretty much shut off fuel delivery when you’re coasting. In such cases it doesn’t really matter what the RPM is because gas is not being used. Also, some newer cars (again, not sure if yours is one) have grade logic built into their transmissions, which means when you go downhill they’ll automatically downshift if necessary to keep the car at the proper speed.

The “two characters” in question rarely visit the forum. I’m afraid you’ll have to settle for the cast of characters (including myself) who hang out here. The good news is that some of us are engineers, some mechanics, some scientists, a chemist here and there, and a host of other physics-minded folk.

The bad news is that we’ve debated this ourselves, and with non-hybrids that don’t employ fancy technology to shut the engine down we don’t all agree.

The RPMs are only part of the picture. The air and fuel entering your engine’s cylinders is being pulled in by the pistons going down on the intake strokes. It would seem logical that the faster the pistons are going, the more air is being pulled in and thus the more gas would be drawn in. But it isn’t that simple. When you take your foot off the pedal, the inflow of air is restricted by the throttle plate. Therefore, the air beyind the plate is far lower pressure and density, meaning few oxygen molecules per volume. The amount of gas fed by the injector is determined by a combination of all these factors, throttle plate position, amount of air coming in, manifold pressure (vacuum), engine speed, and, finally, the amount of oxygen that comes out the exhaust manifold. Whether the ECU feeds more gas in with the foot off the pedal at 2000 rpm with high manifold vacuum or the foot off the pedal at 700 rpm with less manifold vacuum is, I suppose, dependent upon the engine as wlel as the drivetrain setup (automatic, 4-speed, 5-speed, or whatever).

I’m not sure this is a “one answer fits all” question.

Others here, will, I’m certain, disagree. Like I said, we’ve had this debate before.

SiL’s more correct - many cars shut off injectors now, and even if your’s doesn’t, it’s the better way to come down the long hill, better control.

Sounds like a smart SiL!

My first post addressed the gas mileage question only.

But I agree with the others here that on a lengthy decline you’re “favorite son” has given you good advice. Using a lower gear provides better control and it helps prevent the brakes from overheating.

Your son is a smart son.

Some newer cars (not sure if your Scooby is one of them) pretty much shut off fuel delivery when you’re coasting.

I have one, but remember it cuts the fuel off based on the RPM's   If you are going down hill in gear, you likely are not using any fuel.  However if the RPM's go below idle speed, it will feed enough fuel to the engine to maintain the idle speed.

The RPMs are only part of the picture. Very true. The “correct” answer is a bit complex, but if you keep the engine above the idle RPM, then with most of those new cars, you will be running 0 fuel usage.

In real life this only happens occasionally and your action will only improve fuel mileage a little bit.

Engine braking only works if the RPMs are high enough to retard motion. If the car is still idling, it isn’t in a low enough gear.