[userbinator]

When taking into account wind resistance, the optimal speed for efficiency varies greatly with the current weather conditions too: with a tailwind, the optimal speed goes up, while the opposite is true with a headwind.

Usually the speed at which you first shift into top gear and cruise comfortably is close to optimal

Although automatic transmissions and lack of tachometers are probably responsible for this, I've noticed that a lot of drivers remaining at the upper RPM range for a certain gear while cruising, when they could've sped up 2-3MPH and upshifted. I don't know if there's a term for this, but it's certainly recognisable as a passenger: the engine is much louder than it should be, often with accompanied higher levels of vibration and discomfort.

[vlasev]

Over the long term you expect the weather to average out and cancel itself out of the calculation. The question is does the headwind lower your efficiency more than the equivalent tailwind raise your efficiency?

[PeterisP]

Yes, in general getting 50/50 up+down hills, as tail+head winds decrease your total efficiency, as if you drive both directions (getting both the advantage and disadvantage), then you tend to drive the disadvantaged half slower (but not using less power, so no efficiency benefits from that) = it affects you longer = it has more effect on efficiency.