It is common practice to drive at constant speed as much as possible to reduce fuel consumption, or at least to avoid "pumping" on the throttle (at frequencies high enough that the lowpass filter does not absorb it) but why? 

Obviously accelerating then braking is just a waste of energy through heat, but what about simply stopping accelerating (since engine losses are there whether one accelerates or not)?

**Application example**

You're on the slowest lane just under the limit, when you get closer to a truck in front of you. You can't overtake so you have to stop acting on the throttle and lose 20mph with the engine brake alone waiting for an opening. When it's clear, you accelerate a lot to get back to cruise speed; then repeat the situation several times. Why is it inefficient?

**Potential solutions:**

1) Similar behaviour of the internal combustion engine to electric DC motors where the (Joule) losses are proportional to the torque. Why is that so?

2) ?