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)

 

thanks so much for the kind replies

in response to the prompts, so far:

Drives are mostly highway, typically 20 miles or more.

I got the NGK plugs from OReilly - briefly considered and firmly rejected getting from Amazon out of concern for counterfeits (if I replace again, probably Rock Auto or some other reputable place with no 3rd party sellers)

tire pressure is 32 but I'll only fill to 35, as spec'd (I am not trying to game mileage, just figure out why it dropped)

fuel blends did not change during the noted time (E10 summer blend all the way)

weather ... I'm in a coastal area. Temperature variation is fairly low between seasons

The tires are supposed to be LRR, but are an updated version of one I had before and had no trouble with (I think the car has gone the requisite 1000 miles break-in, too - it seems to have recovered mileage marginally and may gain noticeably more when I get the pressure corrected)

I had forgotten about the vacuum hose trick (had never needed it before) - I plan to acquire the correct socket before ever pulling the plugs again (the only reason I did last time is because I had the cowl removed and considered myself committed - should have just reinstalled the cowl yada yada)

not tried, yet:
After first posting, I noticed another thread that features a long list of possible diagnoses, including some I had not considered. One that caught my attention was the 12v battery (I'm still on the one that was in the car when I rolled it off the lot 10 years ago). I plan to check that, soon. I'll take another look at that list, as well