Ok i'm confused i heard that, and common sense that if you roll with your windows down you get higher MPG, however i also read the it reduces your aerodynamics therefore reducing your MPG. If that's the case then why my following tests show these results. These results are based on the same usual driving habit about 50% highway / 50% City, and filling the tank and once the pump stopped dividing the number of gallons consumed by the number of miles driven.

First test = 28 MPG with A/C on and ocassionally windows and moonroof open 1/2 tank remaining when going to the gas station.

Second Test= 31 MPG with A/C off (Windows and Moonroof opened at all times) 1/4 tank remaining when going to the gas station.

Third Test = 27.26 MPG with A/C on at all times and windows never down 1/2 tank remaining when going to the gas station.



Filled the tank up today and was surprised to see i got 27.26 MPG since the last time i pumped. If the aerodynamics thing is true then why these results?

 

Because it's not true.

Either way though, I just roll the back window down... I don't have aerodynamics problems. lol

 

well, the a/c puts quite a strain on your 1.7l I4. the power loss is more or less the reason for the worse mileage when using A/C.

 

Okay so the Mythbusters tested this and found that there was almost no difference between the different combinations. So I'm going to have to go with them and say that it doesn't matter, and you are going to get around the same gas mileage no matter if you use the a/c or not, or if the windows are up or down.

plus, you are doing this in normal driving conditions, so you still have many variables in your equation that are not accounted for. These vary from the differences in how fast you accelerated, to how many times you had to stop and start at red lights, to how much time was spent on the highway versus city driving. The list could go on, but the reality is that under normal driving situations, there is no effective way of measuring the differences in MPG.

The only way to do it is like what the Mythbusters did and do the test on a closed circle track where they could drive the same exact speed for a full tank of gas. This takes out any of the variables that I listed above, and will give the best results. And as I said earlier, they found it to be no difference in MPG

 

As Sig mentioned above, if your speed, stops and starts, weather conditions, routes, etc. for each fill up differed, as they likely did, then you are comparing apples with oranges, thereby rendering your mpg experiment invalid.