my truck has a disturbing trend. in the summer my mileage is around 23-24 MPG. in the winter it goes to 17-18 MPG. my driving habits don’t change, that I notice. someone told me it was “winter gas” but I have trouble believing a 20-25% drop in MPG is normal.
what can I do?
it might be partially “winter gas” if they have lower-energy-content (read: ethanol) gas in the winter and not in the summer.
But it’s also pretty normal for mileage to drop in the winter anyway. Cold air is denser, which means the engine computer will dispense more fuel to match the extra oxygen content. More fuel = worse mileage.
The good news is that until it starts snowing, your car is a little faster, too 
It’s more than just the winter formulation of the fuel. It is also the cold weather. Cold air makes it worse. It’s s combination of factors.
It’s normal. It’s the cold. Your engine runs richer when it’s cold, the colder the richer. And it runs richer longer. Mine drops too.
but a 20% drop?
I have owned 4 cars in the last 25 years, this is the only one that had a noticeable drop, and it is so much…
While I admit 20-25% seems a bit much, fuel mileage always goes down in the winter. Winter gas formulation is part of the reason, but the cold weather accounts for most of the reduction. The computer enriches the fuel/air mixture when the engine is cold, and it takes longer to warm up to normal operating temperature in the winter than in the summer, thus burning more fuel.
Short trips, or a short commute to and from work, is the worst-case scenario and would likely produce the largest reductions in mileage.
If you drove primarily on the highway over long distances the reduction would be less, but you would still get less mileage in the winter than in the summer.
You didn’t tell us the age of your Tacoma, or how many miles it has accumulated. Perhaps there is another reason for such a big difference. What’s the maintenance history of your truck? Have you consulted a mechanic about this?
Headlights, heaters, wipers, defrosters, stiff lubricants, stiff tires, 5 minute warm-ups, slower moving traffic all conspire to reduce mileage in the winter time…
How often do you check your tire pressure in the winter time versus summer time?
From what I remember, for every 10 degrees the temperature drops, you lose 1 PSI in your tires. So, you could be driving on under inflated tires and not even know it, which will contribute to lower fuel mileage.
Age and mileage on the thermostat? I’d replace if over 5 years / 50K miles.
An old, sticky thermostat can interfere with warm-ups in cold weather.
Some areas of the US do change gas formulation in the winter. Depending on where you live that can be part of the decrease in mpg. If you drive primarily short distances then that is the rest of the lost mpg.
Colder weather means more natural rolling resistance and more inherent resistance in the mechanical parts of your drive train. Cold tires don’t roll as easily. Cold wheel bearings don’t roll as easily. All the fluids in the differentials, transfer case, transmission, and motor are cold and thicker than when warmed up. All this contributes to making the truck use more power to attain and maintain speed. After about 20 miles all these fluids are warmed up, so on a long highway trip you might get close to the same mpg as in the summer. If your normal drives in the winter are 20 miles or less, you will see a decrease in mpg.
Thermostat and cooling system condition were exactly my thoughts as well (in addition to everything else that was said).
How much do you let the truck idle during the winter? That will make a huge difference in your mileage.
I usually let it warm up around 2-5 minutes…
and the change is like flipping a switch, one week I get 23MPG, the next 18. and it stays that way till spring, when it is again like flipping a switch.
No real need to let it warm up that long. You get better gas mileage, and the engine heats up faster, if you drive moderately within a minute of starting up. Quicker warm-up = better mileage.