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Improving gas mileage

A friend who drives a Mazda 3 automatic, told me that she saves on gas by shifting to neutral every time she stops (stop sign, red light, traffic jam etc.). Could that be true? By shifting so frequently between neutral and park, could something go wrong with the transmission?

If anything she’s using more fuel since the engine will idle at a slightly higher RPM in neutral than drive. At any rate she’s already driving a fuel efficient car to begin with. She is putting extra wear and tear on the levers and linkages, and some extra wear on the transmission, but not a great deal. Over time the extra wear and tear may add up and bite her back. I wouldn’t do it.

Likely she does save fuel. Likely it is about 0.01 mpg overall. Modern cars will use less fuel when stopped in neutral than in gear, but not much. The advantage of being in gear ready to move far outweighs the minimal fuel savings. BTW does she leave it in gear when slowing up at an intersection? If she shifts out of gear when approaching an intersection, she is using more fuel not less in most modern cars.

The driver of a car can make a bid difference in mileage, but there are some tricks that are just not worth worrying about.

The real problem is that she is stopping a lot. It’s amazing how good your gas milege improves if you just run those red lights.

(Just kidding, I was being facetious, calm down.)

On older cars with carbs and a fixed idle throttle opening, I think that leaving it in drive actually reduces idle fuel consumption, the lower engine rpm means lower manifold vacuum which means less air gets sucked past the throttle and that means less fuel gets burned.

On newer EFI cars and diesels, the idle rpm is kept constant meaning the computor automatically adjusts the throttle and that means keeping it in drive burns slightly more fuel as does keeping the AC on.

If she really wants to see her gas mileage go up dramatically, tell her to stop accelerating towards red lights so that she doesn’t need to sit there idling for so long.
I see it every day, people flooring it and then nailing the brakes and for what? To be one car ahead at the next red light as I catch up with them again just letting my car coast.

Every time you shift in and out of drive, all of the drivetrain components take a jolt. These components are designed to take this impact, but shifting in and out of drive accelerates this wear and could lead to premature failure of one or more of the drivetrain components. However, something tells me your friend won’t listen to you or our advice. So relay our warnings, and then when she has to have her transmission rebuilt, you can say “See! I told you so!”

We get this a lot. Most vehicles, not including the old 2 stroke SAABs, are better served as all have noted, being driven the way the manual recommends. Leave the darn thing in gear !!!
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PS…like Whitey said, she may not change. DON"T LET HER DRIVE YOUR CAR.

Not true. It’s not just RPM, it’s RPM x torque. In neutral it takes slightly less gas to run the engine slightly faster than when under a greater load.
(The savings is not noticeable, and it’s not worth the trouble.)

While it does put more wear on the linkage which could catch up with her in the long run, the drag on the engine in “D” when stopped (disspated by the torque converter) in a modern car is extremely minimal. She’s fooling herself about the gas savings. By the way, I assume she’s going from D to N and not shifting in and out of park.

But hey, it’s her car. It isn’t a dangerous practice, so let her do it.

As we have printed on our license plates in NH, “live free or die”.

Yeah…we Mainiacs always wondered about that saying on your plates…do you actually think you’re going to get out of here alive ?

Nope. But while we’re here we hope to be as free as possible at least as regards anything we do that harms nobody else.

At least that’s the idea. We’re losing the battle, but we keep the dream alive.