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The question of whether or not to downshift a manual transmission has been addressed, but what about automatics? I personally always downshift when coming to a stop, and my latest vehicle (Ford Focus) has a Tiptronic-style gear selection mode that I use regularly.

Note that I am referring here to a non-emergency stop in clear weather and level ground. Of course I downshift on steep declines and I personally prefer to hold a lower gear when in challenging driving situations.

Is it ill-advised to downshift an automatic transmission in normal driving?

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What is the specific mechanical question that you're asking (vs. just a question about driving style)? –  Bob Cross Jun 7 '12 at 17:11
    
Thanks, Bob, I clarified. –  dotancohen Jun 7 '12 at 17:13
    
I'm interested in the answer, even though I hate automatics. I'll sometimes downshift them to lay off the brakes on a descent, but that doesn't imply that one should/shouldn't do it. –  Ehryk Jun 8 '12 at 0:23
    
How is this question different from this one? mechanics.stackexchange.com/questions/1695/… –  Bob Cross Jun 8 '12 at 17:57
    
Thanks, Bob. That questions specifically asks about hills, while this question specifically asks about everyday driving on level ground. –  dotancohen Jun 9 '12 at 7:49

1 Answer 1

up vote 6 down vote accepted

First, let's start by listing some facts that we know about the brakes and the engine:

  • The brakes in a car are designed to stop the vehicle, both in emergency situations as well as under normal driving conditions.

  • The automatic transmission in modern automobiles is optimized to keep the engine running at speeds that balance fuel economy with engine output. If an engine runs at too low an RPM or too high an RPM, this output is reduced, and the engine will consume more fuel as a result.

  • The 2 and 1 (or L) on the selector lever in an automatic transmission, and the lower gears on the Tiptronic gear selector lever, are designed to give the driver more control over the vehicle in certain circumstances. For instance, 2 is recommended when starting on snow or ice to avoid spinning the wheels, and 2 or 1 are recommended when driving down steep grades to prevent premature brake failure.

Here are the reasons why a driver would need more control over the transmission:

1 - Safety: Prevent brake failure

When traveling down steep hills, the continuous pressure applied to the brakes in order to maintain speed causes the brake pads to heat up. Using engine braking on steep hills is a good idea for the following reasons:

a) save your brakes a lot of unnecessary wear & tear
b) actually HAVE brakes at the end of the hill!

It's important to note that the main reason we use engine braking on downhill slopes is to prevent the loss of arguably one of the most critical safety systems in the vehicle, the ability to stop! In other words, in this situation, we trade some wear and tear and loss of fuel economy in exchange for protecting the braking system.

From Engine Braking Pros/Cons:

When driving in a mountainous area, braking continuously during a long downhill stretch really heats up your brake components. In this case, it makes sense to alternate between using the brakes and engine braking. If you actually smell your brakes heating up, you should definitely engine brake. If possible, pull off to the side of the road to let the brakes cool off.

2 - Safety: Prevent loss of control on slippery terrain

As with braking, running the engine in a lower gear is done as a safety precaution on slippery terrain. Once again, we trade wear and tear on the drivetrain in exchange for decreasing the risk of loss of life, limb, or property, which could occur if the operator loses control of the vehicle and careens into a person, another car, or some other object that creates immediate damage.

Summary

While engine braking, whether it be in a vehicle equipped with an automatic transmission or manual transmission, is primarily used for safety reasons, your specific question is more geared towards whether or not using engine braking as a standard approach to stopping the vehicle is a sound idea.

Saving money?

The main reason people engine brake is to save wear and tear on the brake system. Using the engine to slow the car can mean longer lives for brake pads, discs, rotors, and other expensive brake system parts.

However, increased revving puts added strain on a car's drive train — not only the engine and transmission, but also parts like U joints. Repairs to any of these parts are far more costly than brake repairs. So in the long run, most experts agree that engine braking will probably cost more than using the brakes. One argument against engine braking is that it wastes fuel because it causes the engine to rev up. However, this applies mostly to older cars with carburetors. For newer cars with fuel injectors, the increase in fuel consumption is minimal.

If you approach this purely from a financial standpoint, it doesn't make sense to use engine braking in your daily driving. The amount of wear on the brakes, under normal driving conditions, is negligible, but the tradeoff is more wear and tear on potentially more expensive components.

Additionally, we know more fuel is consumed when the engine isn't running at it's optimum speed for power production.

In summary, engine braking won't do any immediate damage to your vehicle, whether it be automatic or manual. But while it makes sense to use engine braking for safety reasons, it doesn't necessarily make sense to do so for purely financial reasons considering the increased costs and transfer of ear and tear to the engine, transmission, u-joins, and other components involved in powering the vehicle.

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I had no idea that engine braking consumes more fuel in a fuel-injected engine. I had assumed that with the throttle closed the additional RPM would not be met with an increase in fuel delivery. Thanks. Also, I agree with every bit about drivetrain wear. –  dotancohen Jun 8 '12 at 7:55
    
The excessive fuel consumption was news to me as well. On my motorcycle, I discovered that it ran more efficiently at higher RPM's. Every engine has a different optimum range, so on the bike, "engine braking" is a bit more normal than it would be in my car. –  jmort253 Jun 8 '12 at 8:00

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