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So, to put it a little differently, on low-grade hills I will generally leave my car in 5th and climb it under high load, near peak BSFC, then shut the engine off and coast down the other side. This allows me to maintain a constant speed, which is more efficient (in this particular case) than shutting off the engine at the bottom of the hill, coasting up and arriving at the top with near zero speed, then accelerating rapidly under power down the other side.
It takes the same amount of energy to push a car up the same hill (minus wind resistance) at 25mph as at 50mph, the difference being that at 50mph, you're using up that energy twice as fast. That's why you perceive it to be harder on a bicycle - you're applying twice as much force, but for only half the time, for the same net energy used.
Let's say the hill is shallow enough that you can keep the engine in an efficient BSFC range, and you want to average 40mph:
1) You can climb at 40mph under power, then shut off the engine and coast at 40mph down the other side, and average 40mph.
2) Alternately, you could come at a hill at 80mph, shut off the engine at the bottom and coast up, hitting ~0mph at the top, then accelerate under power down the other side of the hill to reach the bottom again at 80mph, ready for the next hill. In this case you would also be averaging 40mph.
In both cases you use the same amount of energy to lift the mass of the car to the top of the hill, but in the second case you have to overcome a lot more total wind resistance, because it goes up exponentially with speed. For this reason, pulse and glide over gently rolling hills can often be even more efficient than pulse and glide on level ground.
Problems do arise when a hill is steep enough that you need to shift your engine into lower BSFC ranges. Although maintaining a constant speed is more efficient in terms of total energy needed, you're right in that you often more than offset those savings if you have to downshift several times. I'd say in a majority of those cases, it's best to leave the car in its efficient range while accelerating up the hill and allow yourself to bleed speed, rather than rev it up and lose efficiency. For a hill this steep, you're almost certainly going to need brakes going down the other side to avoid speeding though, and any time you have to use your brakes, you're turning potentially useful energy into brake dust.
You should almost never be accelerating under power down a hill.
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