just curious on what brand of all-season tires you got? I switched to Hankook Optimo 727 last november and the difference is night and day between OEM. I'm still getting 34-36mpg so I don't think I lost that much, I have to wait after winter if I could still achieve the same mileage with these new tires. All-season tires should have the rain/cloud/sun/lightning/snow icons and with M+S (mud+snow) mark on it, I've read good reviews from owners in MN that's why I went with Hankook.
Info is here on why the inspection mode is not meant for driving (2 ways to activate it, one with the intelligent tester and for those without that equipment "the dance" is used manually)
http://techno-fandom.org/~hobbit/cars/idle.html
Also it's stated in the toyota service manual not to drive with inspection mode active.