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I need to get a mileage estimate for my Civic now, to compare it with what I got last fall. Problem is, now in SoCal we have a heat wave, and I've been running the AC almost nonstop when I drive. Last fall I doubt I used it once (Oct-Nov). I usually run it only on the first level, but occasionally on the second.

Is there any way to accurately guess how much of a mileage hit I can credit to my use of AC? For what its worth, I drive pretty conservatively, 65-70 on the freeway. Last fall, I got 28.5 MPG combined, so the car normally gets pretty good mileage. I don't have a recent pure city or pure freeway MPG estimate.


Has anyone here measured an MPG hit when they run the AC? Or is trying to factor in an accurate drop in mileage impossible for me to do--rendering my mileage comparison worthless?

 
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There's definitely a difference, when the compressor engages there's more load on the engine.
Mythbusters did an episode where they tested the difference between windows up and ac on versus windows down and ac off.
I would do a week of driving with the a/c always on, and then a week with it off.
Start each week with a full tank, it will give you a rough idea

 
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OK, but I gotta say that week with the AC off ain't happening till a few months from now.


I was hoping to get a decent idea of my mileage with the AC on, by the end of this month.... I'm beginning to think that I just don't have enough data to go on. Perhaps I really will need to run the Civic without AC to tell. I was wondering if folks here with their 6th Gen Civics knew already.

 
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I use a fuel mileage app called fuelio, I just put in information everytime I fill up (odometer, amount of fuel, and total price) and it keeps track of everything and calculates fuel mileage and stuff. You could try that, it will take a few tanks before you get a good average. It will generate graphs and charts too.

My girlfriend drives the 6th gen that we have ('99 Sedan) and she doesn't keep track of the mileage

 
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Consider aerodynamics (wind drag or Cd) of most modern cars on the highway
70 MPH with windows up and AC on, wind drag is usually less than the same speed windows down and AC off, so it's more efficient to run the AC.



In town/stop and go, aerodynamics doesn't really come into play.



It's 90+ here, I want my AC on full blast while I go home from work, and IDGAF about my in town mileage when it's hot out

 
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We were hotter than Jamaica a couple weeks ago, the AC was struggling but still worked.

 
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Holy mackerel. I just gassed up. This tankful got me 27.1 MPG combined; last fall I clocked in at 28.5-28.8 MPG. I can hardly believe that I got such good mileage--there was maybe 50 miles more on this last tankful that were 50-65 MPH than the earlier tankful; also, I used Techron a couple of tankfuls back. Still, given the fact that this tankful was with the AC going, I can hardly believe it. The AC couldn't have hurt my mileage too much.


I realize that there are a few variables here that were different, but I'm just thinking that with an expected drop of 3 MPG because of the AC, that my mileage was going to be a lot worse. Perhaps the Techron helped?

 
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around 65MPH is the sweet spot for mileage?

since your thread is not about performance modifications, moving to general section