[moonfire (OP)]

With heat condtions as extreme as 100 degrees and more in Texas, is it the same consumption of fuel with air conditioning or not?  I had a discussion with someone on this today about the compressor and the pull on the engine.  Also, would it make a difference with the consumption of petro when the refrigerant is low?

[elegantlywasted]

It depends on how fast you are driving. Mythbusters did some tests, I beleive if you are under 65 mph then roll the windows down but over that speed turn the AC on... I will look for the link to that episode

[paul.fr]

I think this was discussed on TNS, i think they said windows down below 70 mph and use aircon above. Very close to megs answer.

[kdlynn]

does the heater make any difference

[moonfire (OP)]

It depends on how fast you are driving. Mythbusters did some tests, I beleive if you are under 65 mph then roll the windows down but over that speed turn the AC on... I will look for the link to that episode

If we are under 65mph and it is 100-110degrees farenheit roll our windows down....erm, I think people who drive in Texas would have road rage worse than it is...our normal highway/freeway speed limit is 60-70mph(legal limit)but generally drive 70-85mph....The heat is very humid and with our traffic congestion, pollution, and multiple factors it would be safer...LOL  Please post mythbusters info as I would like to look at it further and these are accurate tests based on what type of vehicles?  Or was it only one type of vehicle?

[moonfire (OP)]

Will it make a difference as well if the refrigerant is low in the air conditioners for the vehicles?

[elegantlywasted]

This was taken from http://kwc.org/mythbusters/2004/11/mythbusters_boom_lift_catapult.html

Windows down vs. air conditioning
"Urban puzzle": it is more efficient, on a hot day, to run with the A/C on and windows up than to run with windows down (b/c of increasing car's drag).

Computer-based mpg measurements:

11.7/11.8 with A/C on and windows up
11.7/11.8 with A/C off and windows up
11.3 with A/C off and windows down
So, according to the computer, it's better to use A/C with windows up.

This was too quick and easy for TV, so they decided to stage a seven hour marathon, race-til-you're-empty duel, with Jamie driving an SUV with A/C on and Adam driving an SUV with windows down. Though, once the safety inspector intervened, it was no longer a seven-hour marathon, it was a bit slower (45mph instead of 55mph), and a lot shorter (only 5 gallons each).

Jamie's A/C car ran out of gas first -- Adam's windows down SUV ran for another 30 laps -- completely contradicting the computer mpg estimate. Computer estimate based on air flow into the engine, so it would appear that it is unable to properly model the difference between A/C and windows down.

Here are some other links:
http://www.newscientist.com/backpage.ns?id=mg18925432.200
http://en.wikipedia.org/wiki/MythBusters_(season_3)#AC_vs._Windows_Down
http://www.straightdope.com/classics/a2_393.html

[moonfire (OP)]

The fundamental flaw in the MythBusters' test was that the point where the drag becomes powerful enough to inhibit a car's performance with windows down was inside their 45–55mph margin at 50mph. Going less than 50mph it is more efficient to leave your windows down, but going greater than 50mph it is more efficient to use your A/C. Since then, this particular bit of knowledge has been used frequently on Car Talk.

This would vary I am sure depending on the car model/type.

[elegantlywasted]

Definatly... unfortunatly i do not know enough about cars or physics to help you further []

[another_someone]

As you say, it will depend on how well the a/c is maintained, as to how efficiently it works.

It will also (as implied but not stated by Lo) depend on the air temperature.  Air temperature will have three effects: firstly, if the outside air is too warm, then winding the windows down may fail to cool you at all; secondly, in warmer air, the air density is lower, and so aerodynamic drag is reduced; thirdly, in theory, a car engine will improve its efficiency as air temperature reduces (although this theoretical aspect may be tempered by specific trade-offs that might have been made in the engine design).

The other factor to take into account is the airflow within the car itself (if you can maintain an air pressure within the cabin of the car that is lower than the outside air pressure, then this too might help cool the cabin of the car).

[moonfire (OP)]

Thank you Meg!  Georgie, thanks!  I guess flat, straight, curvy, or hilly roads, altitude, etc....too many variables to get an accurate measurement...

[diverjohn]

I used to manage an auto repair shop. Here's my thought on the AC question:
old AC systems (1960s and 1970s) were inefficient, heavy, and would noticeably slow the motor when activated. Modern AC systems are smaller and much more efficient at moving heat from the passenger compartment.
I believe that if this question were asked 50 years ago and a couple of Chrysler Imperials were driven on the highway (one with AC on, the other with windows down), the results would be different.

[chris]

While driving across the Kimberley in Western Australia, the fuel consumption meter on my 4x4 said I was burning about 14 litres of diesel per hour with the aircon on. When the range got a bit dicey I turned off the aircon and the fuel burn dropped to about 12.7 l/hr.

This was a Mitsubishi Pajero, so quite a big beast; perhaps the compressors on smaller vehicles are not so demanding...?