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.
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?)
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.
Bailey Approach 740
And I can't wait to get on the road again.
GoinŽ places that IŽve never been.
SeeinŽ things that I may never see again
I haven't actually done a proper check on the last 2 vans we've had, too depressing really, and I'm more concerned with the overall cost and how many miles I get from a full tank, I drive at a sensible speed 50-60 dependant on conditions, no hard acceleration, and avoid braking at all if poss, anticipate the traffic lights etc, stay in a high gear as long as possible but avoid labouring the engine, clutch and gearbox.
I try to get 400 ish from a full tank in the van, 600+ in the car, sometimes I get a bit more, sometimes less.
I use Road Trip LE on my phone (free part of the app) to keep tabs on the fuel used and mileage readings. It then shows me graphs of average mpg and fuel consumption. The mpg varies a lot on each trip, but the average value is not that far from what the computer in the van says.
I never even bother looking at the computer readings, the only way is brim to brim.
Driving styles considerably alter the mpg, rushing away from traffic lights, heavy braking, stop start. I always try to drive smoothly and try to keep the wheels moving, I hate having to stop at traffic lights!
Speed, gear changing, motorways or national roads all contribute, along with hills, or just moving the van a few times to go to the borne, and weight!
Just drive as smooth as you can!
While I use the instantaneous reading to verify what I'm doing as a guide, I drive exactly as you describe. Additionally on long rolling hills, I'll accelerate on the downhill and maintain on the ups.
I use the Eco app on the Garmin sat nav. Set it up with the details of the MH ie avg fuel consumption around towns & motorways. When I stop to refuel I check how many ltrs the Garmin says I should need and find its very accurate. This also tells me my mpg.