E36 Air Conditioning and Fuel Efficiency

[MikeB325i]

I just saw a news report about the "myths" of improving your gas mileage per gallon. One of the tips given was that on a highway trip, you'll get better gas mileage if you use your AC instead of driving with your windows cracked and/or sunroof open (which supposedly increases drag). Is this true?

I thought that using your AC (no matter what type of car you drive) drastically cuts down on your fuel efficiency/miles per gallon. Is this true? Is there a difference between using your AC in stop-and-go driving conditions, as opposed to using the AC at a constant speed on the highway? Any suggestions?

I've calculated my mileage per gallon ('93 325i) to be around 25 miles per gallon (on highway trips, with intermittent AC use). Is this good/bad or average?

Thanks in advance for any tips. Gas prices are getting out of control, and my commute to work is 150 miles (one-way)!...Luckily I only have to make this commute about once every other week--Otherwise I'd be broke!

Take care,
-Mike

 

[Big Daddy]

The answer to your first question is yes, having a window or sunroof open causes drag and far more than the A/C does (on the highway). The answer to your second questions is it depends, yes in stop and go driving the A/C will cause more reduction in fuel economy than on the highway, it has to work harder to keep the car cool. According to the US government the fuel economy estimates for your car were 19 - 28 (with a manual transmission) mpg, that would be under ideal conditions. Has your car been tuned up recently? That can make a big difference. Have you used any fuel injector cleaner recently? This too can help.

 

[MikeB325i]

Big Daddy--Thanks for the info!

Yes, my car was tuned up recently, and right after it was tuned up I ran a bottle of fuel injector cleaner through the fuel system. I didn't notice much of an immediate difference--But I'm assuming that you probably have to run the entire tank of gas through the engine in order for the fuel injector cleaner to do it's job thoroughly.

How exactly does the AC work? (Why does it put such a strain on the engine, causing such a noticeable power loss and lower fuel efficiency?)

I guess if the estimated fuel economy for my E36 (which IS a manual) is 19-28 mpg under ideal conditions, I'm doing pretty well! I get almost exactly 25mpg (again, highway driving at a fairly constant speed of 75-80mph, with intermittent AC use). I get around 20mpg in "stop and go" driving (with the AC off the entire time).

How accurate is the "Mileage Meter" (the needle that supposedly shows how many MPG you're getting)? I try to maintain a consistent speed that gets me the best MPG--According to my "Mileage Meter". Should I really trust this thing?

On another note...

One thing that strikes me as rather odd: My 'Check Engine' indicator light has been on for 90% of the time since I bought the car (two years ago now). Up until a few months ago, I'd always use 89 octane gasoline or higher, and sometimes this would seem to solve the problem (sometimes the indicator light would NOT come on, if I used premium gasoline and/or octane booster)...

Once I started commuting from Albany, NY to Boston, MA for work (around May), I consistently started using "regular" gas (87 octane) in order to cut down on my commuting expenses. Now that I'm using regular 87 octane gas, the 'Check Engine' light doesn't come on at all anymore! I haven't noticed any other adverse effects either (no knocking, pinging, power loss or anything of the sort). Can anyone explain this one to me? Maybe there are additives in the higher octane gasolines that somehow trigger the 'Check Engine' indicator?

Have any of you had a similar experience after switching to lower octane gas?

Thanks again for any and all advice. Have a great night everyone!

Take care,
-Mike

 

[Quiksilver_7]

IF YOU GET A K&N PERFORMANCE AIR FILTER YOU COULD GET UP TO 10% MORE MILES PER GALLON. I HAVE ONE IN MY 92 325I AND I GET 30-32 GOING 65MPH AND ABOUT 26-29 GOING 70-75. ITS ABOUT $35, BUT I SHOULD PAY OFF OVER TIME. OH AND THANKS FOR THE WINDOW DRAG TIP, MY A/C IS OUT AND I HAVE TO CRACK OPEN A WINDOW.

 

[jrt67ss350]

In my E30, I have calculated that I get 1 mpg better on the interstate with all windows up and the A/C on than with A/C off and the windows down and sunroof open.

 

[Kirby]

Myth Busters did a study on this, they tried several techniques. If I remember correctly they got conflicting results depending on how they did the test. My interpretation was that it's darn close either way, but windows closed with A/C is slightly more efficient.