The next fill was somewhat less than 42 mpg for a variety of reasons but still average or average +. What I did learn, though, was that even on the "closed" setting and 90 deg ambient temps, that the e-fan doesn't run excessively. I was planning on making an e-fan "on" indicator lamp but my test, that of coming into a town on that 90 deg. day from a long highway run, going their 30 mph for a bit, then getting out to listen for the fan showed that the fan was always off. Unless of course the a/c was on, but the e-fan is always on for that.
I could very well go even smaller on the opening but I want enough airflow to ensure the a/t stays cool and that when the a/c is on the e-fan is easily able to draw outside air; it looks so small now I may just leave it. If the car had a stick I'd be more inclined to try going even smaller.
I also have seen much reduced warm-up times from cold start.
P.S. To clarify, there are no idling warm-ups under my watch; this is from cold start then driving off even more gently and slow vs normal until the temp gauge gets in the normal range. The temp gauge got into the normal range far sooner.