I think I got a bad tank of gas at Wawa, which is a bummer, because I love that place. I filled up and soon after, my car was having a hard time turning over. I thought it was possibly my battery at first, so I went to WalMart. The person had no idea what to do with the tester. She couldn't figure out how to read it and thought it was dead, so I got a new battery under warranty. It didn't change anything about the hard starting after I put the new battery in. Eventually, I figured out it could be the gas. I had already gone through the majority of a tank. Filled it up, and no more hard starting. The problem is that my gas mileage went from 17-18 mpg to 15 and change. This is not only on my display, as I also checked by tracking mileage in between tanks. It is not my tires. I just put on new tires this past week and they have plenty of air. I have 102,000 miles on it since I bought it new. Any suggestions, for what I should do next? The two things on my radar are MAF and spark plugs. I would love if you guys could give me some input on what should be tried first or a list of things I should try. I appreciate any help!

Toyota recommends, right in the owners manual, that you use Top Tier Gas (Consumers). Top Tier Gas has added cleaners and other additives over what is the minimum government requirements. You can find a list of gasoline brands that are Top Tier gas at: https://www.toptiergas.com/licensed-brands/

I don't see Wawa on the list, so you probably got junk gas there.

If you got a tank of bad gas, it may take several more tanks to flush all of it out. Until then, the bad gas will likely cause lower mpg. You also state that you got new tires. If your new tires are a lot heavier than the ones they replaced, you may see lower mpg. I’d give it 3-4 more fill-ups to get rid of the bad gas before I worried too much about it.

Hmm you got a few variables there.
What exactly is your timeline of events?
How many tanks have you gone through since your suspected bad gas?
Did you use any fuel system cleaner since this started?
How old was the battery? Original?
I’m assuming plugs are original?
Do you perform your own service?
Any services you have skipped? Diffs and T/C? Air filter, Oil change?
Have you cleaned the MAF and throttle body?

Hopefully you’re right and it was just some bad gas.

If it were me and I couldn’t answer these questions, I’d be checking everything. I would start with all four of my new tires and wheels for any issues: correct size, load rating, pressures. I’d also jack up each wheel to make sure they were installed correctly and the lug nuts weren’t overtightened. I'd check the bearings and see if the wheels spin freely and smoothly. I’d check the hub temps after driving.
I would hook up TechStream to check any codes.
I’d also check the battery and charging system and my serpentine and pulleys.
I would definitely pull the plugs and check/read them. At that mileage I would replace regardless.
Yes, I tend to overreact.

I have put about 7 tanks of gas in it since the original hard start began. It has been roughly 2 months. I used fuel cleaner for the 2nd and 3rd tanks after I realized the fresh fuel erased the hard start. I have never used fuel cleaner before in this vehicle. That was the 3rd battery for this car. The plugs are original. I do all my own maintenance by the book since 30K after my free maintenance period was over. No maintenance skipped. I am pretty diligent about that stuff. I have never cleaned the MAF. Also, I know it is not the tires, so don't focus on that. I am going to look into ordering some OEM spark plugs today, but still looking for more advice. Thanks!

If you use Top Tier gas exclusively, it has all the cleaners you need; you do not need to use any additional fuel cleaners or additives.

Spark plugs are a good idea. Although with modern cars, spark plugs can go 100K miles, they may need to be changed earlier.

While you are checking/cleaning stuff, clean the throttle body as well.

Have you checked for trouble codes? A dirty MAF wil often trip a code from the upstream oxygen sensor. Also, is your vehicle included in the fuel pump recall? As for spark plugs, when you pulled them, did they all look the same, and relatively clean? Always buy your engine parts from a dealer. There's a lot of bootleg, fake parts on Amazon and ebay, etc. Hope it all works out!

Could be the tires, are they the exact same size? Unless they’re identical replacements they could be heavier than the ones you replaced, plus fresh tread vs. worn?
Edit sorry I missed where you said it’s not the tires.

Frustrating. If the 2012 doesn’t have the smog pump then a plug change is easy. I changed mine a little early and I’m glad I did, color was good, no oil, but the electrodes seemed a bit worn.

I have found that mileage is quick to drop and slow to rise. Zero out your MPG and give it a month. Also, what tires did you get / Size / PSI running? Three things changed: battery, bad gas, tires. I would start with those three things before anything else.

If you had the battery out for more than 15 minutes then your ECU will be relearning your driving habits and engine nuances and re-tuning. That will mess with your mileage as well until settled.

Keep us posted on what you find.

did not know about this until right now. good info, thank you.

I was under the impression that I would need a check engine light to come on for me to have a trouble code. Is that not correct?

My 2012 is not a part of the fuel pump recall. I remember checking that the day I replaced the battery.

I know it is not the ECU reading of mpg because I reset my trip meter every fill up. It is a habit of mine. The drop in mpg has happened by my manual calculations and the mpg reader telling me so. So that can be checked off the list.

I know it is not the tires because these are the same size and brand that I just ran for 48,000 miles. The bad mileage has continued with the old tires ran with 37psi and as low as 30psi. I adjusted to fully wear out my old tires. I run them pretty stiff normally. The bad mileage has now continued with the new tires. Since the bad mileage continued with old tires, new tires, high and low psi, that can be checked off the list.

I ordered OEM spark plugs that I will receive on Monday. I will change them. I don't believe that the stuff in the way was installed until the model a year or two after mine (2013 or 2014). My plugs have never been pulled before, so I don't know what they will look like. After I change them, I will report back, but keep the suggestions coming with what I should do if that doesn't work. Right now I have MAF clean and throttle body clean on the list. Thanks!

I changed the spark plugs today and it fixed my problem. I topped off my tank and checked the mileage on my last 90 miles.... 14.3 mpg. Yuck. Drove close to 20 miles after fill up and averaged 19 mpg. Now that is more like it! The rear driver side plug looked noticeably different, in a bad way. I will try to upload a photo soon.

So the plug that looked different was cylinder 6, which I have read seems to be a normal thing. I really didn't look bad, it was dry but just dirtier. Hopefully, I can get pics up soon.