Well, the good fuel economy effect was discovered on insightcentral that you over filled the tank spilling fuel into the characoal canister and it helped to enrich the mixture of air into the engine, so it used less fuel from the injectors and the system read higher than normal mpg.
When my father bought gas for me I always over filled my car and never notice any of this. I just like to tease my former co workers that I can go 175 miles before the needle moves off the full mark.
