When I need AC, I start set it on AUTO and "LO" on my 2007 XLE and once the cabin becomes comfortable, I set the thermostat to AUTO and about 75 degrees.

Does running the AC at 75 degrees save a bit more gas than running at 65 degrees? I would think so but wanted confirmation.

 

Because you have automatic climate control, I personally cannot be certain how the computer runs it. However, on a normal vehicle w/manual climate control, changing the temperature will not make a noticible difference.
The reason being, turning the temperature dial closer to the red zone, simply opens up the heat valves to mix with the a/c. The a/c compressor etc. are still running normally because it is designed to become as cold as possible at all times and does not recognize the user controls.
A simple test would be (for manual controlled cars), to turn the a/c on and turn the dial to max heat. You'll notice the a/c fan and compressor are still running, and condensed water will continue to drip to the ground.

With automatic climate control, i would believe the same is true, but maybe the computer is smarter and shuts off the compressor when not in use - which is what i'm assuming it does. So with auto climate control my guess is that the higher the temp, the more fuel you'll save - but running the fan itself also lowers your mpg very slightly.

 

Good question. My first guess is that it makes no difference in MPG regardless of set temperature. My second guess is that it makes a small difference (e.g.about a1/2 MPG difference).

If you don't get a definitive answer here, or you just want to compare answers on different forums, post this same question on the cleanmpg.com forum (http://www.cleanmpg.com/forums/).

 

I think that the laws of thermal dynamics would suggest that there is a difference. However sooo tiny tiny that it would not make a noticable change. I'm going with what nodrogkamsaid. It would have to have a computer to identify the position of the climate control interesting question. I will be happy to see what others post!!

 

Unless I am mistaken the auto temp controller simply mixes a variable amount of warm air with the cold air to maintain the temp you have selected. The compressor continues to run (cycle)to maintain a specific a/c system pressure. No savings in fuel.

 

So you say that the a/c cycles based on pressure and not temp?

 

That was not the best explaination. Sorry for giving you incorrect info but the principle is the same. There are max and/or min pressure switches onmost a/c units. These switches protect the system from damage caused by over press and low press due to lack of refridgerant. The system comppressor generally cycles based on a reading from a temp sensor that is just slightly downstream of the evaporator. Warm air for cabin temp is mixedfurther downstream of this point. This is controlled either by a manual or auto temp control. So the compressor cycles on temp but not cabin temp.

Now let me say that some of the later model vehicles may actually take a cabin temp reading and makes aninput to a climate control computer that determines compressor cylces. But as you know when you cylcle the compressor off momentarily you generally will get a face full ofhumid air. By letting the compressor maintain a relatively constant temp at the evaporator you will continually keep it cold enough to condense water out of the air. My thought is that a cabin temp controller alone would not be sensitive enoughto provide timely adjustments to maintain a comfortable cabin with respect to temp or humidity.

 

Got it (mostly) but let me run this by you. If the the evaporator is cooling the cabin and a certain rate at full blast, (rate based on the temp of air passing through it and how much) would the the system cycle less if the warmer air was added after the evap? I mean if the evap suddenly did not exchange as much airwould it not have to work less?

I really hope I just made sense!

 

in different terms, if the air is cooler around the evap would the r134 evaporate less easily than if it was hot air around it?

 

The colder the air going into the evaporator case the colder the exiting air will be. A properly operating system will drop the temp approximately 20 degrees as it passes over the evaporator. The temp sensor located immediately downstream of the evaporator will cycle the compressor off at a temp a little above freezing, maybe 35 -40 degrees F. Google "vapor cycle cooling" or "how air conditioners work" for some good (better) explainations of how a basic system works. Although the vapor cycle systemin a home and a car work the same the reality of its workings are different. For one your home is better insulated than your car. The a/c can cycle on and off and keep the house cool maybe only running 1/3 to 1/2 of the time. The car loses the cool or heat extremely fast so the heat or a/c need to operate pretty much continuously to maintain a constant temp.

The only way to make the system cycle "on" less would be to: a)reduce the airflow over the evaporator - it does not have to absorb as much heat, or b) start with cooler air to begin with (use recirculated air). Now if your temp control computer uses afan speed to keep a constant temp (some do) thenafter the car has cooled to acomfortable temp and the fan slowsin order to maintain that temp the compressor would not have to work as hard. If you turn up the temp as you said several things may happen. If the fans slows to low the compressor will still cycle enough to keep the humidity down and the car will warm due to normal heat gain principles. Once the temp stabilizes the fans would pick up to maintain the temp. Your temp control may add hot air to bring the temp up also. If it is a manual system the air is just heated.

Your big question is about higher temp settings and better mileage. I don't think that you will ever see any difference by using the method you mentioned unless you monitor it over a long period of time. You might save a penny or two per tank.