If you track your car’s gas mileage, here in McHenry County, you’ve noticed that your mileage generally goes down; you just don’t get as many miles per gallon in the winter as you do in the summer.

Why is that? One of the primary reasons that winter gas mileage drops for drivers bearing the full force of a McHenry County winter is that the cold affects a change in the way we operate a vehicle from the start. In other words, we tend to start the car and let it warm up before we drive.

If you allow your car to warm up more than a minute before driving, you’ll probably notice that doing so has taken a bite out of your winter gas mileage. It’s not a bad idea to warm the car up before you drive. The engine likes this. It allows the oil to warm up and flow better before you start putting the engine under a strain.

Some people, however, will start their cars and let them run outside while they go about getting ready to go inside. A car that idles outside for 10 or 15 minutes is burning gasoline the entire time. You can’t help but notice a drop in mpg.

Another factor that affects winter gas mileage is less obvious. In the cold, the air pressure in tires falls. This is a matter of pure physics. Things that are hot expand and things that are cold generally contract. That applies to the air in your tires, too.

With lower than specified air pressure in the tires, you guessed it – your winter gas mileage takes a hit.

For many people with aluminum rims, they may have noticed this phenomenon already because they’ve come out to the car on a cold morning and discovered they have a flat tire. No, it’s not necessarily a nail in the tire; it’s just that the cold contracted the air too much and, lacking adequate air pressure, the tire separated from the rim. The solution is to bring the tire into a trusted auto repair shop where they can clean the rim and reset the tire with a good seal.

There are some who suggest that using the defrosters or seat warmers can affect winter gas mileage. This is questionable. However, if you like to leave the car running while you’re in the store, so it’s nice and warm when you return, the entire time you’re in the store, your car is using gasoline and the winter gas mileage is suffering as a consequence.

Another possible cause of lower winter gas mileage is that the oil companies sell a different blend of gasoline in the winter. This winter blend is based on the need for the gasoline to atomize in the cold. The winter blend is more volatile and, therefore, atomizes easier to help the engine run. Whether this has an effect on winter gas mileage is debatable but possible.