So... can this be a reduction in drag under the van?
I know that fitting a front spoiler to my van has made a huge difference to how well it drives at motorway speeds, there is much less tendency to drift and shimmy about and it seems less affected by the wake of lorries passing as well so the airflow under the van must make a big difference in stability. I doubt that it would make that kind of difference to the speed and economy though, much more likely that there was a bit of muck in the fuel lines or carb or something which was restricting power that has now cleared.