Why is 'hard' braking bad for fuel economy ?

[basjoos]

That was a typo. Change those GPM to GPH. It was still early in the morning.

[Cd]

My city MPG average is currently between 35 - 37 MPG. I only drive 7 miles one way to work, coast when I can, have memorized the timing of the lights, and hardly EVER use my brakes with more pressure than it takes to crack an egg ... err... an ostrich egg.

My mileage sucks so badly because I am driving an automatic.

On the highway, I have seen over 54 MPG at a steady 55-60 MPH

According to my SuperMID, I loose between 2.5 to as much as 4 MPG when stopping at a light. ( On the instant average readout )

I was just asking the question about braking out of couriousity. Thanks for the feedback.

[TestDrive]

From 100+ hypermiling / ecodriving tips to increase gas mileage - EcoModder.com

49) Conserve momentum: brake hard

It sounds like a contradiction, but there are rare times when braking hard can save fuel compared to coasting or light braking: it's a "damage control" technique when faced with an unpredictable/unanticipated stop or slow down ahead and not a lot of space.

An example: approaching a fresh red traffic light that had no other indicators to predict the change (no pedestrian signal and no cars waiting on the cross street). If you brake lightly/moderately, you will cover the entire distance to the intersection and have no option but coming to a full stop.

But if you brake quite hard initially, you can potentially scrub enough speed and buy enough time to coast the remaining distance to the intersection at a low speed. With judgment and some luck, you'll arrive at a fresh green light and avoid a full stop.

Obviously, rapid deceleration isn't a safe option if there is following traffic.

[MetroMPG]

Hey - that's the exception!

[TestDrive]

Originally Posted by Cd

According to my SuperMID, I loose between 2.5 to as much as 4 MPG when stopping at a light. ( On the instant average readout )

On re-reading, I find myself stumbling at the words instant average. Seems like it would be one or the other. Does the manual for the SuperMID give a description of what instant average means?

[Katana]

I do use engine braking a lot, all you do is coast in your current gear or shift to a lower gear and the car slows quicker, you have to be sensible and not shift too low and rev the engine too high.
I asked my father about this as he's a mechanic and he said the engine doesn't use fuel while engine braking on a fuel injection car, since when you coast in gear it turns off the injectors, but the pistons and the other engine stuff is still moving so it still sounds like it's running although quieter.
Are we both wrong?

[The Atomic Ass]

Originally Posted by TestDrive

On re-reading, I find myself stumbling at the words instant average. Seems like it would be one or the other. Does the manual for the SuperMID give a description of what instant average means?

I'm not sure about the SuperMID, but on the cars I've driven that had an instant average, it calculated a rolling average over the last mile, so that the number is fairly accurate, but not jumping around like the instant display.

[dcb]

Originally Posted by Katana

I asked my father about this as he's a mechanic and he said the engine doesn't use fuel while engine braking on a fuel injection car

sometimes. In my metro if I coast in top gear above 40mph, it will kill the injectors till it gets down to 35mph, per the mpguino and the butt-meter.

[Tango Charlie]

Katana, you're describing DFCO; Deceleration Fuel Cut Off. (Let's not miss an opportunity to use an acronym!)
My Vibe does this down to about 1200 RPM. It's a more efficient option to using the brakes.

And yeah, I was trying not to muddy the waters with tip #49.

[The Atomic Ass]

Originally Posted by Cd

Also, can you explain engine braking ?

I notice when coasting to a stop I will shift my automatic from neutral back to drive, and the car will shudder and suddenly slow as the engine 'catches ' and revs up.

Is this an example of engine braking ?

That is *NOT* an example of engine braking, it is an example of "how to break a transmission." Any time the car is shuddering, it indicates something is wrong. Some cars take to being shifted in and out of drive while moving, others, like yours, do not. You'll have problems down the line if you continue what you're doing, I guarantee it.

Engine braking involves letting off the throttle. Yes, it's that simple.