Rick's Free Auto Repair Advice

Bad gas mileage in winter

Why you get bad gas mileage in winter

Gas mileage always drops in winter and there are very good reasons for that. The four main reasons you get bad gas mileage in winter are:

  1. Cold engines rob heat from the combustion process
  2. Winter fuel contains less energy, so it takes more winter gas to achieve the same performance as summer gas, and
  3. It takes more gas to heat the engine up to operating temperature when outdoor temps are low.
  4. Cold engines create more friction
  5. Generally, you use more electric power in winter to operate your window defogger, heater, heated seats. You also run your headlights longer in winter due to the longer nights.

A cold engine reduces gasoline vaporization.

Engines don’t burn liquid gas. Instead, they burn gas vapor. In summer, the fuel injectors spray the liquid through small nozzles to form fine droplets. Then, the heat created during the compression stroke, along with the heat of the combustion chamber, causes the droplets to change state from a liquid to a gas.

A cold engine, on the other hand, quickly dissipates the heat created during the compression stroke, robbing the droplets of the heat they need to change state.  To combat that problem, refiners convert to a “winter gas” in late Fall and that winter formula includes more volatile components like butane and propane. Butane evaporates (changes state from liquid to gas) at just 33°F and propane evaporates at -44°F, so they evaporate and ignite faster than summer gas.

A cold engine quenches the combustion process

Once your engine fires up, but before it’s up to full operating temperature, the cold metal reduces combustion efficiency by quenching the burn before it’s complete. So it takes more gas to keep your engine running and that translates into lower gas mileage.

Winter gas contains less energy per gallon

While the highly volatile components help a cold engine start and run smoothly, they don’t contain as much energy as summer gas which contains a lower volume of those components. On average, winter gas has 1.7% less energy than summer gas.

More of the gasoline energy is used to heat the engine to operating temperature

Engines operate most efficiently at around 200°F. So it takes more energy to heat a cold -10° engine up to 200°F than it does to heat a summer 90° engine to 200°. And, while a summer engine may cool to around 125° while you’re at work during the summer, that same engine will cool all the way down to -10° on a windy blustery winter day. So you’ll spend all that energy again after work to re-heat the engine back up to 200°. In winter, every time you stop to go to work or go shopping you’ll be spending more gas just to get the engine back up to operating temperature.

Oil thickens in cold weather and that creates more friction

What more can I say? Cold oil creates a drag on the engine until it heats up to operating temperature. So it takes more energy to rotate your engine when the oil is cold.

You have to generate more electrical power in winter and that takes gas

There’s no free lunch here. You run your heater, defogger, seat heaters and headlights more often in winter than in summer. That means more electrical power, which means more gas.

What you can do to improve winter gas mileage

1) Don’t let your engine idle to warm up. It’s a waste of gas and your engine will heat up much faster if you drive it than when you idle it.

2) Turn off electrical accessories sooner. I get it, you want your seat heaters on as soon as your buns hit the seat. But do you really them once the heater is blowing warm air?

3) Reduce the number of short trips in extremely cold weather. That reducdes the number of “re-heating” cycles on your engine and that saves gas.

©, 2020 Rick Muscoplat


Posted on by Rick Muscoplat

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