Such questions are extremely treacherous. They can be answered at various depths of analysis. Choosing to stop at the wrong depth of analysis can give you a disastrous answer. Politicians who are too clever and not clever enough demonstrate this year in and year out.

So imagine you had a driving community in which a sizeable majority saves fuel by accelerating responsibly, but the usual quota of munchkins, who would have accelerated wastefully anyway, throw conniption fits and assert their freedom, responsibility, competence and, you should excuse the expression, mature manhood, by accelerating even more wastefully. We can't have that can we? So all the rest of us begin to accelerate wastefully as well to avoid waste?

Sorry, I don't have any facile surefire solution but I suspect that quantitatively we can do better by driving as considerately and responsibly as possible, thereby influencing at least the majority of road users favourably, than by all flocking to join the stampede of the Munchkins.

Of course I might be wrong; I have been wrong before. It was fun that time too. :-)

Cheers

Jon

In pure physical terms, it takes the same amount of energy to accelerate a given weight to a given velocity no matter what the rate of acceleration.

However, modern cars (and their powertrains in particular) have many design features which complicate things. Most cars are designed to run more economically under low load and at low revs and have power-boosting features such as variable valve timing or forced induction which become active under high loading or when engines revolutions are allowed to climb. Obviously the more power an engine outputs, the more energy is required to create it. Automatic transmissions are also very smart these days and will react to more or less agressive driving styles to extract optimum performance and/or economy from the engine.

So in summary, your associate is almost certainly saving his own fuel bill with his current driving style. However, numerous studies have shown that traffic flow, average speeds and road safety are directly impacted by differential speeds between vehicles. So if every hypermiler and boy racer went an little more with flow, we'd all benefit.

Your friend should be reminded that the public roads are not a laboratory, and driving so slowly as to impede other's progress is very selfish (and actually illegal).  Obviously this is subjective, but being overtaken 6 times in 10 minutes would (should) get even a learner pulled over.  If they want to accelerate efficiently and legally, the following might help:

Try the obvious things car-wise, ie a diesel or hybrid (diesels are better at efficient acceleration).  At all costs avoid older autos

If they have a petrol, use full throttle to accelerate but change gear early (keep the engine between 1-3000 rpm, with the ideal 1.5-2000 for acceleration).  don't accelerate slowly but have to stay in lower gears as a result.  I use this and easily get 50+mpg, beating the car's official mpg, and don't annoy anyone.  If they have a diesel, the same rules apply for revs but they are less sensitive to throttle openings in terms of efficiency.