Why has my gas mileage dropped?

I drive a 2014 Ford Focus ST. I recently reset my dashboard stats and noticed my average mpg has gone way down. So I reset it again and did some slow driving. I used to get around 31 mpg highway and now I'm getting 24. Tire pressure is fine.

I would get it check out!

Winter.

Do a tune up on it, run some injector cleaner thru it too.

"Tire pressure is fine"? Presumably you check the psi of all 5 tires once per week? When was the last tune up performed on the engine? Last trans fluid/filter change? Those will affect the mpg quite a bit

Before you go spending big bucks just take a look at the air filter. A dirty air filter will cause a drastic drop in gas mileage quicker than most any other component and they're a cheap fix which you can DIY.

Check gasoline quality--some stations add water for bigger profits.

I'm wary of that "recently" business. You can't judge anything on the basis of one or two tanks. And besides, it's December. It's cold out there.

Don't trust the on board mileage guesstimate. Fill the tank and write down the number of gallons you put in. Then set a trip odometer to zero. When you need gas again fill the tank and divide the number of miles you drove by the number of gallons you just filled up with. That's the only way to get a good mileage reading.

Has there ever been a check-up performed on the vehicle. Usually when fuel mileage falls so rapidly, there's a sensor that's bad somewhere in the system that may be making the fuel system run on the rich side. If a sensor is bad on the lean side, the engine will usually stumble under acceleration and fuel mileage will actually be slightly better. There are computer modules you can buy that are much cheaper than having a dealer connect their computer to your vehicle, or have a 'back yard' mechanic take a look.

Your spark plugs are responsible for sparking combustion in your engine. If they misfire, or are working poorly, this can affect your gas mileage in a negative way.