+1 on cold weather. More energy required to push through denser air.
Have you been starting the car and letting it idle for a bit before driving?
Do you find yourself sitting in traffic more?
I found my travel time increased this winter significantly.
OVer the last year, my mileage has gone from 17.6 MPG to 16.2 MPG, with the same style and amount of driving, and even oncluding a tune-up in that time frame.
However, the composition of the hydrocarbon content in gasoline does change, for two reasons, environmental requirements and economics.
Environmental requirements change from winter to summer, in several respects. But the an important one for mileage is the reduction in light components in gasoline in the summer. these light components tend to cause evaporative emissions and are severely curtailed in summer blends. Light components have fewer pounds per gallon than heavier components. Mileage measured in miles per gallon, is a function of the pounds per gallon of fuel. On the other hand, winter temperatures make the gasoline heavier per gallon than summer temperatures, which offsets this to some degree.
Economically, when gasoline at the wholesale level is worth more than diesel fuel, refiners tend to put some of the molecules that could go into either fuel pool into the gasoline pool. When diesel is worth more, refiners tend to go the other way. These molecules are generally heavier than average gasoline and increase the density of the blended gasoline when they are diverted to the gasoline pool. These heavy molecules are most often from the back end of the gasoline component from a fluid cat cracking unit.
87 octane gasoline generally contains a higher percentage of heavy molecules from the fluid cat cracking unit than does 93 octane gasoline. this increases the pounds per gallon in 87 octane gasoline, which seems to offset the impact of the anti know device “de-tuning” a car running on lower than the manufactures recommended octane. For the past twelve years I have experimented with both 87 octane and 93 octane gasoline in the same car for 1000 miles at a time. I usually achieved about 0.1 mpg higher mileage on the lower octane gasoline, even though the Lexus involved was recommended to use higher octane. Statistically, that is probably not significant. I did not detect any knock.
Density of gasoline can be determined with an API Hydrometer. Unfortunately, this is a bit complicated and requires several hydrometers each calibrated for different density ranges. It also requires a correction to a standard temperature. Hydrocarbon density is generally expressed as “API Gravity,” with a lower number indicating a more dense fluid than a higher number. Here is a link to a Wikipedia article on API Gravity. Included at the bottom of the article are links to hydrometers, instructions for using the hydrometers, and comments on the API adjustments required for temperature. http://en.wikipedia.org/wiki/API_gravity
If you run the defroster a lot, it will reduce your mileage because the Air Conditioner comes on when you run it.
My car is an old isuzu beater. When I filled up in Oklahoma the car ran great, got about 320 per tank, was surprisingly responsive. When fill up in ethanol states (no choice in most of the country and hilariously, Texas gas stations have big banners bragging about ethanol) the isuzu runs like one cylinder shut off and gets 280 per tank. Also, the “check engine” ligt is always on with ethanol fuel. As soon as a fill back up with pure gasoline the ligt goes out. THANKS GOVERNMENT FOR ALWAYS KNOWING WHAT’S BEST FOR THE NATION!