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2014 Sienna LE
305 Posts
Fun fact: Air density leads to fairly significant MPG changes for highway driving.

Drag force is linearly proportional to air density. Air density changes 10% from 40 °F to 90 °F. All else being equal, this means that at 40 °F your air drag is 10% higher than when you’re driving at 90 °F. Exact proportions of air drag to other losses vary from vehicle to vehicle, but some searching showed that at highways speeds maybe 75% of vehicle energy is used to counter to air drag. If this is so, then if you got 35 MPG highway driving at 90 °F, then the same speed highway driving at 40 °F with the same gas would give you a MPG of 32.4. Humidity also changes air density, but going from 1% RH to 99% RH at 90 °F changes air density by 1.8%, so humidity plays a much smaller role in air drag changes than does temperature.

The air density causing drag to vary therefore is likely the biggest single factor by far of any MPG changes that are not related to changes in driving habits, and the numbers I quote here do line up with real world experience I saw with my non-hybrid Sienna used for primarily highway driving during the drastic temp swings I experienced this spring. My driving was identical so I started researching this because I noted my fuel economy varying drastically by temp, which could not be explained by summer vs winter fuel blends, since it didn’t seem the energy content of the gas would change that much. Nor was it possible that the gas was switching back and forth from summer to winter blends multiple times exactly in sync with the weather. If anything they will change blends twice a year, once in spring, once in fall.

Not saying this is what the OP saw, unless they live in the Southern Hemisphere and experienced a winter cold snap. More likely, as mentioned, driving patterns changed, using more fuel, so the car adjusted the estimated mileage to match the new fuel usage. If so, the longer mileage estimates will return when the driving usage returns to previous levels.
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