I remember reading, I think on this site, about a comparision of driving with windows down.... botton line driving was that driving with the windows down didn't effect gas milleage.Tyler_Canada said:...to turn off the vent and open the windows... if I'm driving slow of course.
It depends how fast you are going. Bottom line, going slow, open windows. Going fast, use AC.Gonzo said:
I remember reading, I think on this site, about a comparision of driving with windows down.... botton line driving was that driving with the windows down didn't effect gas milleage.
Since using the AC has a "fixed cost" in terms of load on the engine (the compressor is either on or its off, no in between) there is probably a speed where its more efficient to use the AC than keep the windows open. Mythbusters on Discovery channel revisited the segment they did on this "myth" and concluded in an informal study that the speed where this crossover occurs is around 45mph. Not completely accurate, but good enough for me.Gonzo said:Well I think that is just it... even going 75MPH it didn't have much effect... these are cars not jet airplanes. You raise the gear on a small single engine at 150 MPH you only gain about 1-2 MPH.....
Yeah, Mythbusters did it very badly the first time. Quite simply, A/C uses a fixed amount of horsepower, while the drag from having the windows down requires an exponentially increasing amount of horsepower as speed increases. At some speed the power required to overcome the wind resistance will be more than the power required for the A/C, and it's probably between 45mph and 60mph.Eric L. said:
Since using the AC has a "fixed cost" in terms of load on the engine (the compressor is either on or its off, no in between) there is probably a speed where its more efficient to use the AC than keep the windows open. Mythbusters on Discovery channel revisited the segment they did on this "myth" and concluded in an informal study that the speed where this crossover occurs is around 45mph. Not completely accurate, but good enough for me.
I like it quiet when I drive so I can hear my music very clearly, so I tend to use the AC all the time anyways. Also prevents annoying bugs from flying into the car and buzzing around.
Maybe if you drove the exact same stretch of road, at the same temperatures, using the cruise control. That would indeed be a crude but demonstrative test. You can google for hours and hours on this topic though, and read about it.Gonzo said:Here is a simple and not very accurate test... but what the heck.....
Opps never mind... the MO doesn't have instant/current fuel economy readings.
I **WAS** going to suggest drive at a constant RPM/Constant Speed/Constant pedal position on a flat road and take a reading at 45, 55, 65, 75 MPH.... with windows up and repeat with windows down. Compare gas readings.
Would this have worked for a simple experiment?
They did a poorly designed test, and the result was inconclusive. The fact that they revisited the topic shows that they eventually figured that out.kdesvern said:Like these forums often go, it turns into a physics lesson about the aerodynamics of planes and cars, not to mention someone even had the nerve to question the accruacy of the Myth-Busters. (My kids favorite show).