Get your car serviced. Even dirty plugs or misfiring carborettor or computer malfunction can do that.

Colder temps resulting in air that requires a denser fuel charge, combined with gas companies that know this, so, cut their fuel with a higher percentage of ethanol, is why you get lower mileage in the winter. Equipping every new car with remote start, so that people run their cars 15mins to a half hour before driving them has only made it worse

Well fuck you guys! I got 9 mpg towing 4 sleds in an enclosed trailer over a 12000' pass. Running a cool 70 mph today 🤣

Winter blend fuel. Globohomo blend.

Check tire pressure which goes down a lot when it is cold.

GC

Don't know why it dropped that far, but winter gas gets worse mileage than summer formulation. Happens annually. Noticed it first when I had a hybrid years ago.

Winter mileage is always worse than summer mileage.

Edit: my Subaru Outback gets about 27mpg in summer, 22mpg in winter.

If you have changed battery or reset the panel it starts at 0 mpg and takes few days/weeks to build back up to a more accurate average depending on driving habits and road conditions

Outside temperature can make a difference also.

You letting your car warm up more lately? Mine goes from 33mpg in the summer to 24mpg in the winter.

I have always thought that gas would “deteriorate” as they want us to by electric cars. The more broken gas cars, the more EV’s we will buy. No sauce, just my paranoia. :)

It’s definitely a conspiracy

The difference in premium and regular has to do with octane rating. Higher octane means the gasoline is suitable to engines that have higher compression ratios. Higher compression ratios are needed for higher fuel efficiency. It has nothing to do with the heating value of the gasoline, but with its resistance to knocking (compression ignition).

To get a huge drop in mileage such as you've mentioned, I would check the engine to see if something has gone out of whack with its functioning. Or possibly, the supplier has gone to a heavy ethanol-based mixture. Ethanol is chemically like gasoline wedded to water; it has very low heating value. Too much ethanol can be deleterious to an engine's fuel supply system. Either way, you really should take it in for inspection. Fuel economy is a great metric to diagnose the health of your engine.

Not sure what kind of car you drive but most manufacturers say to use regular 87 octane. The exception would be a high compression/performance motor. I've been told by my mechanic that most folks that use premium in their cars are wasting their money. Also might put some quality injector cleaner in the tank once in a while. yep mileage goes down in the winter. Mine sure did especially when it gets down below zero. I'm not a mechanic just going by what I've been told. Hope you get it figured out

Try and buy gas where they don't add corn based additives to the gas. CHS (Cenex) is where I fill up. If I can't find one, I go to Chevron. When I tried to save money, my fuel injectors gummed up while MPG went way down, and cost me more money to clean them up.

The past couple days my farts have been smelling a bit more sulfuric, but I just blamed it on the diet changes.

As to fuel, as others have stated, there are a few things that can cause non optimal mpg that people commonly ignore.

Stuff that you can do without any real mechanical knowledge or tools:

Air filters are cheap and usually an easy end user replacement.

You can try an octane booster/cleaner from Lucas or Royal Purple, I've never bothered with the stuff myself but heard it recommended.

Check your fuel cap, if it seems it's not tightening like it used to, you could be getting some air in with your fuel supply causing some loss of optimization, newer cars will sometimes also throw out an error code for this as well.

Outside of those two simple things you start looking at needing to visit a shop or spend some time talking at the parts store.

If you have a check engine light, you can visit a parts store and they should be able to scan it for you and give you a brief overview but not go into too much depth. If you have an o2 sensor or airflow sensor going out it'll give you non optimal output as well, most shops will just say "it's not a big deal" and most people just decline but it'll fuck with your fuel to air ratio and cause you not to be running at optimum.

Spark plugs/tune up.

Winter weather will sometimes make you run a bit rougher as well depending on your environment. Some folks will switch their oil up to accommodate, but most just run the same shit and quick lube places probably don't even take that into account and just go by whatever their program tells them to use for the make/model.

When you start dealing with classic car folks they'll start getting into about all of Chevron's detergents and yadayada, but as long as you're not dealing with a shady gas station most won't even bother with the fuckery you're implying as they just contract through the major corps who take all the profits from the fuel and are on the hook with federal EIA and EPA laws, the gas stations make most of their money on the convenience store side of things.