2009 xc90 3.2 awd Better gas mileage in the summer but not in the winter WHY?

[MoVolvos]

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I used to use a 15-40W motor oil year round in all my cars. When I used it in the 98 S70 T5 the car would take a little more time to warm up in the winter and not as swift. No running issues just feel weighted and mileage suffered a little.

[E Showell]

Gas for the winter is formulated differently than summer blend to reduce air pollution. That difference accounts for the discrepancy in summer/winter mileage & performance. This is well-understood and you can probably find a decent amount written about it. From what I understand, it has to do with Reid Vapor Pressure -- what we commoners might think of as evaporability. Basically, gas need to evaporate more easily in the winter with lower temperatures so starting in cold weather isn't overly difficult, whereas in summer, we don't want gas evaporating excessively in the atmosphere thereby increasing air pollution.

[chefmarkt69]

WOW thank you all for the answers!!!!!!!!!

[abscate]

Everything expands in the heat of summer so you drive farther on those longer roads. Or does your car expand even more so you get there sooner?

Let’s start a Facebook Group to discuss.