Seriously, just did a bit of a trip over Christmas, and noticed this so frequently on the country roads.
It would be a single lane road, and you'd get people (in all sorts of cars) driving at changing speeds (going from 100km/h down to 70km/h) with no rhyme or reason, then as soon as you get to an overtaking lane - BAM, they're pumping 110km/h for the whole thing. When it ends, they're back to their usual random speed.
It drives me mental.
I always thought that the considerate thing to do is to realise that if someone catches up behind your car - the logical conclusions is that you're going slower than them on average, so the polite and considerate thing to do is to allow them to pass as early as possible.
Does no one else feel this way?