Yea, 81MPG is really odd.
It's not possible that anyone snuck extra gas in but as you mention the next tank(s) will tell if this is even remotely valid.
It will take a while to find out though with that much fuel packed in my car.
Not sure if that's good or bad

I don't think I could take a 20-30MPG "even it out" tank!

BTW the video portion of my project is finished and working on the audio part.
Hope to have it finished this month.