After following a couple of threads where people tried to compare mileage, it occured to me that there was more to the story that nobody had yet mentioned. With the price of gas going up and forecasted to continue going up, I'm starting this thread to address issues which you all believe will help us all save money.

Here's my contribution: While this is especially true in the northern half of the U.S. and Canada, I would think that it applies to everyone. I'm talking about the effects of weather on mileage.

Air temperature and density have a lot in common. The lower the temp, the higher the density. Density is important, because air/fuel mixture is on the molecular level - not volume. I don't remember the exact ratio, but someone else probably does - isn't it something like 7 parts air to 1 part fuel?

That's in molecules, not liters. For those of you who don't know how that matters to regular people and not just rocket scientists, please hear me out.

When the air is denser, there are more molecules per liter of air. More air is being packed into a given area. Because of that, more fuel is needed to produce a combustible mixture.

Engines don't count molecules, only volume - however much air can fit into each cylinder is welcome to go ahead. If the air is colder and more dense, there are more molecules fitting into each cylinder, which means more fuel is needed.

Colder air always means your mileage will go down.

Just like there's a magic speed where you get your best mileage, there's also a magic temperature. For me, this is about 70 degrees F - any colder, and the air density causes mileage to go down. Any warmer, and you start needing to do things like roll down windows or turn on the a/c, which also cause mileage to go down.

When we compare mileage, we need to keep in mind that some of us are driving in that magic temperature range, and others are driving in extreme cold. Because of the wonders of the internet, this can be happening on the same day.

 

When I posted yesterday and got 29.18 mpg, it was with temps at 50* and at an elevation of 8000 feet.

 

Next:

When the oil companies are preparing to raise prices, I've noticed that they try to justify it with things that they claim are out of their control. One of these things is supply when they switch blends.

Most people don't know that the gasoline they buy from the same pump at the same station can be completely different, depending on the time of year. But it is!

Gasoline is blended based on the anticipated average temperatures in a particular part of the country (or, I suppose, the world). Here in Minnesota, there are 4 blends, one for each season. They happen to be so different that you can take a cup of 'summer gas',set it outside in the winter and put out a lit match.

The winter gas is formulated to evaporate easier, because of the lower temperatures. The easier a fuel evaporates, the less..... what? Anyone? Anyone? Bueller????

The easier a fuel evaporates, the less energy that fuel contains. Less energy means, worse fuel economy and lower power output.

Summer gas has the best potential for economy and also power. If you're trying to compare one person's mileage in the winter with another person's mileage in the summer, it won't make any sense.

Oil companies like to use the switch between blends as an excuse for raising prices, because supply of the outgoing blend is going down while they ramp up the refineries for the incoming blend. With oil company profits at record levels, I think that's more of a convenient coincidence....
[sm=bustedsign.gif]

 

Good point!

Altitude has a major effect on mileage and power. Unfortunately, it's a trade, as I'm sure you know (but us flatlanders can't comprehend). Mileage goes up, because the air is less dense. Power goes down, because there's less air/fuel mixture exploding in each cylinder.

 

True winter blend sucks here in MN. Weget like 2 to 3 mpg less with the winter blend gas.

 

Let me add that many states are required to sell fuel blended with up to 10% Ethanol. Ethanol is great in reducing emissions, but it lowers your gas mileage (in some cases by more than 10%).

 

Well crap then! I thought the thing was a rocket like it was. When I go down to Missouri this summer, it ought to really be white knuckle time.

 

For a moment there, I thought I was tuned in to "Bill Nye's The Science Guy".....all kidding aside, thanks for the info.

 

That must explain why I am only gettting 22 in the city. The gas I buy does contain 10% Ethanol.

 

Ya know, years ago we put the A/C theory to the test in my husband's then new Plymouth Champ. We were on the Interstate and road the first tank with the A/C on. Second tank with the A/C off and windows down.
We got better gas mileage by using the A/C because of less wind drag. Now if you are in the city with stop and go traffic, I would think using the A/C would cause you to get less gas mileage.