Poor gas mileage...

[400 dollar yaris]

Hello all,

Recently I've noticed a large decrease in my MPG. I know it is winter time and I expected my MPGs to decrease slightly for things like warm up time and low tire pressue, but I think what I am experiencing is more than this.

I reset my odo trip last time I filled up as a little experiment. Most of my driving is highway. For this tank of gas, I have gotten just under 230 miles. When my car alerted me that I have about 1 gal left in the tank. This is the worst gas mileage I have ever had while owning the car. Before the winter hit, I did a spark plug replacement as well as a coolant change. This was the last maintenance I did before experienceing the lower MPGs, I used NGK laser iridium spark plugs, checked the gaps of all, and made sure they were torqued down correctly. The last ones i took out were denso iridiums that had much larger gaps outside of the spec limits for the yaris with very worn tips, so i was expecting an increase in MPG. Also, I used toyota pink coolant. This weekend, I am doing an air filter clean (K&N) and a seafoam treatment on my next fill up to see if this helps. I have been checking tire pressure frequently and made sure to keep the tires around 32 psi. I still have another 1000 miles or so before the next oil change. I always change the filter and I use pennzoil platinum high mileage. I have checked the car's computer with a code reader and its not giving any codes for anything funky. Any other ideas?

Thank you,
400