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      04-03-2013, 02:44 AM   #8
BillionPa
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Drives: 335i
Join Date: Feb 2013
Location: USA

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2 things happen during winter

1: air is colder, so interooler is much more effective, meaning engine can use more timing advance and get more power from the same amount of a given octane fuel

2: air is colder, so engine takes longer to warm up, using more fuel

additionally for me:

3: winter tires go on, they are thinner, better gas mileage
4: winter tires go on, they have more rolling resistance, better gas mileage cancelled out
5: winter conditions mean driving slower about 15% of the time, gas mileage goes way up, but can go down if the snow is deep enough

since i drive longer distances, 2 is inconsequential, and 1 helps, and 5 generally improves trip econ by 20%

my N54 mileage this winter was identical to that during the summer