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I might not have enough experience in driving techniques to write confidently on DWL efficiency, but it seems that from a purely engine efficiency standpoint, it is the least efficient way to go about driving. The engine is working evenly at a relatively low output throughout the drive.
I think that cruise control fails because it keeps the speed constant on the downhills thus often squandering the period in which the engine can be run very gently—or not run at all—in a speed-losing power zone for maximum efficiency. It seems that the MPGs required to maintain speed is a lot lower than what is required to lose 5mph over, say, 15-30 seconds. By a lot, I mean that maintaining speed I use, for example, 35 mpg, and to lose 5mph in 15 seconds I'm using 60 mpg. Using downhills lets one extend the period of time at which you can spend getting the upper value. By gaining the speed back on the next climb and running the engine in a more efficient throttle zone, one takes full advantage of an opportunity to run the engine.
Why does DWL work?
Does it? Is it simply causing a lower average speed and thusly saving gas?
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