That makes a lot of sense - prevailing wind is normally SW, so going to the tunnel it's cross-ish. But in France & Spain more likely to be driving into the wind (assuming prevailing wind is SW there?)I absolutely agree with all those points, but my observation on the wind was added for the simple reason that M/H drivers don't always realise if they have a strong head or tail wind. Because of this they'll travel a set route twice, using the same driving style at the same speed and wonder why they used a lot more fuel on one of the journeys. This is very often due to variations in the wind direction and speed and may provide an answer to the O/P question.
But let's face it, MPG is a minor consideration, you might as well spend the cash while you've got it. The kids will only crash it on a new BMW or something similar when you pop your clogs.:wink2:
But would you not then expect to generally have tail winds on way back? (I quite accept prevailing wind doesn't mean it's always that direction.). And our mpg goes down (slowly) rather than up on way back...
I quite agree that mpg is not be all & end all, though - I'm just puzzled at variation.