Mileage down since maintenance

My Prius V recently rolled over 200k and I celebrated with a new set of Bridgestone Turanza tires and a new set of NGK Iridium IX spark plugs (yay!)

However, in the weeks since, it's been apparent that my gas mileage has dropped (was about 42 mpg, now about 36-38). There are no engine codes and it seems as responsive as before.

I am aware that E15 is finally approved for sale in my state. However, the mileage decrease started before the governor approved sale (I noticed the mileage change in September and the bill was not signed until October), so if gas stations started selling it already, it would have been illegal.

Basically, I'm down to these 2 candidates for why my fuel economy is suffering. If it's the tires, I'm fine with that and we hope a better design comes out next time (we had a previous generation of Turanza several years ago and did not notice any mileage decrease then - the current set replaced Michelins)

The spark plugs seem more likely (I self-installed and may have damaged them), except for the lack of a code. The computer seems to think everything is fine, except it's burning fuel faster.

Why I think I may have damaged plugs: I didn't have the right socket and neither did either of the local parts shops, so I tried to gently lower the plugs down bare and then used a conventional deep socket to install. I couldn't reach to set them gently so they dropped about a half inch (I fear that may be more than a little for precision components, however). But if they were damaged, shouldn't the engine run rough or throw a code? The computer thinks everything is fine!

Any thoughts which is more likely or whether there is something else I could consider (btw - brand new air cleaner at the same time as the plugs)