If she tampered with your tires, like letting out air or causing a slow leak, that might explain the drop. But anything more severe would probably trigger a warning light. It’s more likely that stress has changed your driving style. Even small changes can lead to big mpg differences.
Check your tire pressure. Tire pressure fluctuates with temperature changes, and under-inflated tires can hurt your fuel economy. I recommend filling them with nitrogen to keep the pressure more consistent.
Has your car’s eco score changed? You might be pushing the gas harder due to stress.
It could be the colder weather affecting your fuel efficiency.
Could be the gas stations switching to winter blend fuel.
Do you smell gasoline when the car is running? That could indicate a fuel leak. Also, how’s the tire pressure? Losing 5 mpg is a significant drop, and it could be worth looking into further.
Someone put a curse on your car, for sure. 
First post here—just wanted to say thanks for all the comments so far!
When it rains, it pours. Stay strong, buddy.
Toyotas can have weird fuel system quirks. My truck fluctuates between 20 and 25 mpg, and I can’t figure out why. Seems normal in the Toyota forums.
It sounds like you’re coping with the stress by driving more aggressively. Give it some time and take it easy. Sorry for the tough situation, but it will pass. Sending you some good vibes!
A 5 mpg drop is a big deal, especially for a new car like yours. Even with aggressive driving or cold weather, the change shouldn’t be that dramatic. You should investigate it further.
It’s weird, but since I bought the car, I’ve actually been getting better mpg in sport mode. Exactly the opposite of what I expected.
Zayne said:
It’s weird, but since I bought the car, I’ve actually been getting better mpg in sport mode. Exactly the opposite of what I expected.
That’s wild! Maybe Toyota intentionally undersold the sport mode’s mpg to surprise customers. But it doesn’t make much sense.