Engine Braking - Questions
 

As my journey to learn more about the world of cars continue on, a question popped up while driving home from work yesterday.

I was falling asleep so I put the car in manual mode (drives an AT) to keep my focus. By the time I got home, I noticed that more fuel was used than usual - I drive in Camry mode when puttering around town.

Was this because of engine braking? I shifted up at 3k RPMs (when the auto shifts) and downshifted around 1k. Whenever I downshifted, I could feel the engine braking kick in. This also happens when in semi-manual mode.

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While I'm here, might as well ask the following - I also drive in heavy stop-start traffic for 30-45 minutes before hitting the highway during workdays. Would the constant, repetitive action of putting the car in neutral>drive>neutral>drive,etc. cause adverse effects to the transmission? I see other people doing it all the time while not using the brakes to prevent the car from creeping (mostly in AT economy cars, of course) so was wondering if that would be detrimental to anything far down the line.

Future thanks!

Okay, I'll get this started and everyone can feel free to jump all over what I say.
Yes, your decreased mpg was most likely due to engine braking. It's a tradeoff you don't usually think about while driving in full auto mode. Do you save gas and increase brake wear by letting the trans disengage (or shift to neutral in MT) or do you engine brake, increase gas usage, and decrease brake wear? My position is somewhere in between to keep driving alittle interesting but also save some gas by not employing heavy engine braking in normal driving and none at all in heavy traffic.
As for switching to neutral and back, I think you'd be avoiding the normal heat generation of leaving the trans in D, but it may inconvenience those behind you and get old as you spend extra time switching in and out of D. I always put my MT car in neutral at lights to save wear on the clutch/throwout bearing.

DFCO - Decel Fuel Cut Off

Your injectors turn off when you decelerate to save fuel. In that condition, momentum is motoring the engine and no fuel is being delivered (until you 1. touch the gas again or 2. enter idle target rpm range). So no, I wouldn't attribute poor MPG to engine braking. If anything, engine braking should improve it.

In a traditional MT car, when decelerating above a threshold rpm, ecu will run fuel cut which will not increase fuel consumption. For an automatic, this situation may be slightly different. I do know that an auto is geared for efficient fuel consumption while it is choosing the gears for you based on your throttle input, speed, and load on the motor. Perhaps your gear selection is not what it would normally be for best mpg.

As to the drive neutral gear change... I'm struggling to see how that is easier or less effort than riding your brake pedal? And are you stopped while switching back into drive?

Edit: beat me to it scott

You probably got worse mileage because you were shifting manually at rpm points that you chose, instead of the transmission shifting where it felt was an optimal point.

As far as engine braking goes, your actual brakes are much more powerful than drag from the engine will ever be. Plus they're designed to be a wearing item, relatively cheap and easily replaceable. Downshift because you'll very shortly need a lower gear, not to slow the car down. Unless you're in snow and ice. Then downshift to slow the car down.

Big thanks to all 4 for answering my questions!


The main street that I use to get on the highway has horrible stoplights. This particular one only lets about 6 cars go before turning red so most of the time, I'm sitting in 5-10 minute bumper to bumper traffic before it turns green again. When I come to a complete stop, I put the car in neutral then let off the brake pedal so I can let my leg rest (injury to right leg a few years back never fully healed). When it's time to go, I apply the brake pedal then switch back to drive, rinse and repeat.

I do make sure I'm not moving before switching back to drive.

When you're stopped, you could try leaving it in drive and holding the brake with your left foot for a few days and see if that changes anything.

XD It'll give my left foot something to do since it.. well doesn't do anything and just sits there.

Anyways, thanks again for the answers guys.

That does sound like an annoying commute... but anyway, meh, you won't experience any noticeable negative side effects so long as you're not moving when constantly changing between drive and neutral... aside from receiving snickers from drivers behind you. I suppose its more noticable when people put it in park instead of neutral so they can tell you cycled past reverse. But ya, so long as you're being safe, I second the motion to put your good leg to better use :)

a lotttttttttta people don't realize this.

so x1000000 for this

For the record, I would NOT advise switching an a/t car into "N" every time you're coasting...a lot of a/t's are not meant to have things spinning when not in "D"

Agreed. That was y I asked if he was stopped when making this gear change or if coasting; affirmed only while stopped. Mexican overdrive is only good on beaters you don't intend to maintain and you know the tranny doesn't have lubricant starvation issues, or so I'm told. I've never ripped apart an AT

Shoulda gotten MT

Ditto

Your engine is nothing more than an air compressor when your foot is off the throttle.