One other thing to check which I came across by accident. The transmission cooler thermostat, if you have an automatic. My fuel mileage recently dropped quite noticeably and I couldn't figure out why. Plugs are almost new, air filter gets changed every 10,000 miles, coils are almost new and the thermostat is almost new. I confirmed the thermostat was working by looking at live data coolant temps and they were as they should be.

So, my expansion tank cracked and when I pulled it to replace it the thermostat for the transmission cooler came apart. So, I put a new one in and my fuel mileage has jumped back up to where it was. It may not be the cause, but it's certainly an interesting correlation. Might be worth checking.