Fuel usage question...

[DQuaN]

So tomorrow is going to be the hottest day of the year in the UK and I have to drive 120 miles to visit a customer. It's going to be really hot so I have two choices.

Have the air-con on (which sucks btw, it needs re-gassing), which will reduce performance and increase fuel consumption.

Or have the windows all they way down.

Having the windows down at motorway speeds will reduce performance and increase fuel consumption!

So this got me thinking, which more so?!

 

[Doog]

Using air conditioning at highway speeds impacts fuel consumption by about 10% versus 20% for having the windows down.

 

[homeforsummer]

Using air conditioning at highway speeds impacts fuel consumption by about 10% versus 20% for having the windows down.

Yup. And the opposite is the case up to about 40mph or so, if I recall correctly. The lower the speeds, the less effect having the windows down has.

 

[Dennisch]

Mythbusters did an episode about this. As mentioned, at highway speeds the aircon is better.

 

[DQuaN]

Using air conditioning at highway speeds impacts fuel consumption by about 10% versus 20% for having the windows down.

Yup. And the opposite is the case up to about 40mph or so, if I recall correctly. The lower the speeds, the less effect having the windows down has.

Mythbusters did an episode about this. As mentioned, at highway speeds the aircon is better.

Good to know. Thanks! 👍

 

[Famine]

Yep. They actually busted their own busting of the myth when it turned out that the lower speed they used for safety reasons was around the crossover threshold - aircon is a relatively constant power sapper, whereas having your windows open increases the drag by a square of the speed and the two meet up around 50mph. Below that, windows open, above that, aircon on.

 

[Beeblebrox237]

I'd like to point out that when Mythbusters did that test, they used a Chevrolet Tahoe, and enormous 4X4 with a big V8. It's not really indicative of anything close to what the average European driver might have. Car and Driver did a test on a BMW 3 Series and found that they lost about 4% more fuel economy with the aircon on vs. windows down at 75 MPH. How scientific their test was, I don't know, but I do know that a big V8 is going to be less sensitive to the added load of the air conditioning compressor than a small I-4, and that a smaller car with less window area might produce less drag. So that's something to consider. Perhaps, @DQuaN, you could do a test yourself, going halfway on each strategy and seeing which one increases consumption more?

 

[Bopop4]

@DQuaN, you could do a test yourself, going halfway on each strategy and seeing which one increases consumption more?

To do a proper test you'd need a scangauge.
Air temperature or tree density can throw things off.



I try to avoid using the A/C or the windows, turn the fan on and then every 5 mintues give the A/C a few seconds to get some cool air into the car and shut off.

 

[hawkeye122]

Mythbusters did an episode about this. As mentioned, at highway speeds the aircon is better.

This (along with everything here) is all theoretically true. Keep in mind that cars have different aerodynamic consequences depending on the model, in addition to the compressor size in each car being varied.