Quote:
Originally Posted by Tcoat
UMMMMM... no.
You are not using fuel when engine braking you are using compression. Extra already explained it perfectly.
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It uses the car's kinetic energy to power the engine. That energy is not free. Coasting down a hill saves it, at the expense of burning a little fuel to idle (hardcore hypermilers shut their engine off when they coast to save this fuel).
Imagine if you were travelling down a slight hill, steep enough to maintain whatever target speed. If you were engine braking, the car would slow down, and you'd have to keep on applying gas over and over to recover speed. If you coast down the hill, you only burn what fuel it takes to idle. If you barely feed it enough throttle to maintain speed, you're essentially idling the engine at a higher rpm, since it's not doing work, which takes more fuel. Apply gas -> engine brake -> apply gas -> engine brake is generally even less efficient.
Nevermind theory, you can try this pretty easily. Reset the average mpg meter on a flat road and turn on the cruise control, note the number. Then reset it, accelerate slowly then clutch in, rinse and repeat, maintaining the same average speed with a bit of fluctuation. The mpg meter will read a little higher if you do it right. The fuel wasted idling is offset by the drag saved by not spinning the engine as quickly and operating the engine at higher efficiency when accelerating.