Outward in my Ford Focus 1.0 EcoBoost 125, not being in a hurry, I took the AutoRoute Calais to Reims and then the Route Nationals (including some town driving) to Colmar where I filled up. My trip indicator said an average speed of 63mph (seems right as I was holding the car at an indicated 65). Miles driven 458 (including UK leg - motorway). Litres purchased: 39.86, mpg: 52.24.

Return leg - Colmar Tunnel to home. Miles driven 483.1 (includes a wrong turning in Nancy and a diversion in the UK as the A2 at Dover was shut). This was mainly motorway and, as I was late for the ferry, I held an indicate 80mph for a couple of hours before slowing to an indicated 70 as I had made up time. Average speed indicated 72mph. Litres purchased 45.22. Mpg 48.57. I suspect some of your contributors to real mpg must wear boots made out of Iridium.