I know it sounds odd. On many days when I use the heater and just control the fan speed, I notice my mileage goes down by 2-3 mpg while following the same driving hablts. I don't have the climate control on. Has anyone had this happen to them?

 

AC ON, thats the ONLY thing that would possibly lower the MPG?? Colder weather usually brings in "blended fuel" as well which gets lower MPG, but heater alone should not/would not do that? I'd think she'd run a lil bit cooler with heat on and get better mileage, save for the blended fuels.

 

+1 I drive a lot in my company car, and it is night and day on mileage between the blended fuel and the decent stuff in the summer. In the summer I can go 55 miles on low fuel warning and when it's cold it will run out at 34 miles. Don't ask how I know.

 

That's what I am thinking too. The AC is only on when the climate control is on right? I could see lower mpg when the compressor is putting more of a load on the car. The whole thing is odd......

 

OK, so try this. Turn the heat on without CC and run her a bit. I'll bet its the AC on CC or the blended fuels?

 

I've noticed the same thing, drop in MPG while running the heater. I know this is probably gunna sound really dumb but how do you run the heater without using the climate control?

 

Article on Winter Blend Gas

Gasoline is composed of many different hydrocarbons. Crude oil enters a refinery, and is processed through various units before being blended into gasoline. A refinery may have a fluid catalytic cracker (FCC), an alkylate unit, and a reformer, each of which produces gasoline blending components. Alkylate gasoline, for example, is valuable because it has a very high octane, and can be used to produce high-octane (and higher value) blends. Light straight run gasoline is the least processed stream. It is cheap to produce, but it has a low octane. The person specifying the gasoline blends has to mix all of the components together to meet the product specifications.

There are two very important (although not the only) specifications that need to be met for each gasoline blend. The gasoline needs to have the proper octane, and it needs to have the proper Reid vapor pressure, or RVP. While the octane of a particular grade is constant throughout the year, the RVP spec changes as cooler weather sets in.

The RVP is the vapor pressure of the gasoline blend when the temperature is 100 degrees F. Normal atmospheric pressure varies, but is usually around 14.7 lbs per square inch (psi). Atmospheric pressure is caused by the weight of the air over our heads. If a liquid has a vapor pressure of greater than local atmospheric pressure, that liquid boils. For example, when you heat a pot of water, the vapor pressure increases until it reaches atmospheric pressure. At that point, the water begins to boil.

In the summer, when temperatures can exceed 100 degrees F in many locations, it is important that the RVP of gasoline is well below 14.7. Otherwise, it can pressure up your gas tanks and gas cans, and it can boil in open containers. Gas that is boiled off ends up in the atmosphere, and contributes to air pollution. Therefore, the EPA has declared that summer gasoline blends may not exceed 7.8 psi in some locations, and 9.0 psi in others.

A typical summer gasoline blend might consist of 40% FCC gas, 25% straight run gas, 15% alkylate, 18% reformate, and 2% butane. The RVP of the gasoline blend depends on how much of each component is in the blend, and what the RVP is of each component. Butane is a relatively inexpensive ingredient in gasoline, but it has the highest vapor pressure at around 52 psi.

In a gasoline blend, each component contributes a fraction to the overall RVP. In the case of butane, if there is 10% butane in the blend, it will contribute around 5.2 psi (10% of 52 psi) to the overall blend. (In reality, it is slightly more complicated than this, because some components interact with each other which can affect the expected RVP). This means that in the summer, the butane fraction must be very low in the gasoline, or the overall RVP of the blend will be too high. That is the primary difference between winter and summer gasoline blends.

Two problems arise from using winter and summer blends out of season. If you live in a very cold climate and tried to use summer gasoline, the lower RVP of the gasoline will make it more difficult to start the engine. Liquid fuels must be vaporized before they will burn. This isn't as much of a problem with fuel injection as the fuel is atomized and sprayed into the piston. On the intake stroke, the pressure in the cylinder is reduced to below the boiling point of the fuel, so some of the fuel quickly turns into a vapor. On the compression stroke there is not enough time for the vapor to recondense. Cold start problems were common on carbuerator equiped engines.

Using winter gasoline in the summer could cause something called vapor lock, where the fuel vaporizes in the fuel lines before reaching the carb or fuel injectors. Again, this used to be a pretty common problem when cars had mechanically driven fuel pumps that sucked the gasoline from the tank before forcing the fuel into the carbuerator. Cars today are equipped with high pressure electric fuel pumps mounted inside the gas tank. These pumps "push" the liquid fuel towards the engine.