A/C question
 

I have just discovered on my new short commute (19 miles) that having the A/C on verses having the windows open is 5 MPG less on the scan gauge. My normal commutes are on the interstate at my top MPG speed of 72 MPH. In the summer during heat waves with the A/C on I got 29 MPG on E0 and 28 MPG on E10 unmoded. I also did A-B testing on the interstate and A/C beat windows down at 72 MPH.

My question is has any one else seen this. My choices on what is causing this 1. the A/C compressor is dying, 2. The slow speeds I run on the commute ( 60 MPH max) makes opening the windows a much less aero hit, or 3. something I haven't thought of. I do have a steep hill to climb at the start of the return journey that may be influencing this different. 30 MPG with windows open, A/C off and 25 MPG with windows closed and A/C off.

You're not telling me enough, so it looks to me like you're comparing apples to oranges. I can't believe you get 28-29 mpg at 72 mph with AC on, and only 30 mpg at 60 mph with AC on. You say AC beat windows down @ 72 mph. How far down were your windows? How high were the AC temperature, duty cycle, and fan set? Was the cabin air set for fresh air intake or recirculating?

My short answer is yes, AC robs mpgs v. windows cracked down 2"-3". The reason is your 2. The slow speeds I run on the commute ( 60 MPH max) makes opening the windows a much less aero hit.

The difference in mpg between my cars at 72 and 60 mph (windows up) is more like this:
You have the capability to test the compressor by driving the same route with & w/o AC. Crack the windows a bit for both A&B (AC on v. AC off) tests if it's a scorching day. Do a 3-5 mile route, traveling the same direction, the same day. Do three tests (ABA). That eliminates different gasoline, different weather conditions, different engine health, etc. Note all the AC parameters. I recommend setting to minimum or medium temperature, maximum fan, and recirculated air for AC parameters. Do it (ABA) @ 72 mph and again @ 60 mph.

Then do ABA testing of windows down 1/3 v. AC @ 72 & 60 mph, and windows down all the way v. AC, @ 72 & 60 mph. That's a lot of driving, but at the end of it, you'll have your answer.

I rewrote my last sentence for clarification.

30 MPG with windows open, A/C off and 25 MPG with windows closed and A/C on.

My cars A/C The 2000 Ford Mustang in my garage has an A/C compressor that is either on or off. I add a switch to shut it of when ever I want to. I usually have both windows wide open to make up for no A/C

Unlike most cars the engine hits 2K at 68 MPH in 5th gear. I cannot shift into 5th until I am at least 50 MPH.

Pre moded summer numbers were like this on the scan gage.

30 MPH - 15 MPG - 4th gear
45 MPH - 22 MPG - 4th gear
60 MPH - 28 MPG - 5th gear
72 MPH - 32 MPG - 5th gear

The 72 MPH A/C numbers on the first post were tank numbers.

I don't have current numbers after the modes. I don't have the money to waste driving around going no where to retest at 72 MPH. For now I will use the A/C only when necessary unless I figure this out.

Dafuq did I just read?

I remember my 99 cobra grabbed some serious air with the windows down. No one could stand to be in the backseat with the windows down. I remember telling others on a cobra forum not race with the windows down, too much extra drag.

My record is still perfect...I haven't turned on the AC yet in my car. Having the windows down doesn't seem to make a great deal of difference at 60 mph (hard to be sure, since there are a lot of hills, so I only go by average, not instant mpg, and I rarely drive exactly the same route twice). With windows up, I estimate the difference between 60 and 72 mph to be about 10 mpg (60 vs. 50mpg) That's really where drag begins to show.

You meant windows open and AC off v. windows closed and AC on? Barring a typing error, it has to be testing variability. There is no logical way you can get better fuel economy with windows down v. windows up at 72 mph in your car, all other conditions being equal.

Yes I messed up again. It should have read:

30 MPG with windows open, A/C off and

25 MPG with windows closed and A/C on.

I just did another test, I realised that I have been using the fan on high which is too cold at 70 MPH but fine at 60. I turned the fan down and got 27.5 with the A/C on even though I was slowed down by slow drivers.

lbar,

There are vents in the back of the trunk that releases excess air pressure. I sealed the rear deck with rubber to make the sound system better. I may have made windows open driving more efficient. Never less it is always better to drive with the windows closed verses open. Especially at race speeds.

I had a short 10 mile trip on the interstate. It was a flat section so I tried A to B A/C verses windows testing. It looked like at 72 MPH that both were dead even. I did notice that having the fan on low freezes you out when at 60 MPH you need high to do that.