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Winter Gas Mileage

A friend has a 2010 Toyota Prius and says MPG is 2-4 MPG less in winter than summer temperatures. My theory is, gasoline expands in warmer weather thereby increasing MPG. Additionally warmer weather maintains tire PSI which increases MPG. What do you guys think?

Well lower temperatures may well result in lower mileage, but not due to expansion of the fuel.  Winter fuel is formulated different than summer.  Less energy in the fuel per volume unit.  Cold temperatures also will reduce engine efficiency and for many drivers will cause loss of efficiency due to letting the car idle to warm up.  

Then you can add the additional energy needed when driving through snow (you have to move the snow out of the way) etc.  

 If the difference was expansion due to temperature, the difference would be far smaller than indicated. 

  Now talking about a Prius and I would guess there is some loose of efficiency of the battery system.

All cars get less mpg in winter. Rolling resistance is higher, cars idle more as drivers keep the cabin comfortable, etc.

In the Prius the difference is even greater as the only way to produce heat is to have the gas motor running. This means more frequent cycling of the gas motor on and off and therefore more fuel is used.

Increased friction due to cold lubricants, more dense cold air to drive through and loss of tire pressure are all factors. Warm up times in ordinary cars which are most efficient at higher temps. poor rolling resistance of winter tires. Probably come up with more that are more important than expanding/contracting gasoline…never heard that one. Have to check the old physics book.

The gasoline engine runs more often in winter than in summer to provice heat for the interior.

Tire pressure is affected by temperature, but the expansion of gasoline has nothing to do with mileage.

The only thing i can come up with is if the tank were far enough under ground, it would take more volume per gallon pumped then in the tank in the summer and less in the winter. So, theoretically, the indicated amount of gas in the tank would be less in cold weather. If the energy content in the entire tank remained constant, your indicated mileage would be better in the winter ? I’m confused.

I’m surprised you only lost 4 mpg. Any car will use more gas in the winter for all the reasons stated.

A hybrid has to use the gas engine more in the winter tp provide heat, and in severe weather it might run all the time.

The sales pitch of hybrids does not cover these situations.

When I stop our Toyota Camry Hybrid at a traffic signal or railroad crossing, I turn theater OFF. Otherwise it keeps thengine running.

I LIKE your higher tire pressure lessens rolling resistance mention. I keep the tires athe same higher pressure all year.

Oxygenated fuels in winter have less Btu/unit and you are likely rolling on water and snowhich imposes much more rolling resistance.

A cold engine is less efficient until it reaches operating temperatures.
Never warm up a car. Just get in, start and gently drive away. It will warmuch faster under load than idling.