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Old 02-25-2016, 06:08 PM   #1
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driving with the A/C on and getting bad MPG

This is something I've noticed before. Being from Southern California, I almost always have my windows down, and don't use my A/C (Because why would I, right?), but the other day I was with my lady and for her sake I had the window up and the air up to 2 but I noticed I had burned through enough gas for me to notice that the needle had moved, and we had only driven about 5 miles and it made me ask these questions:

Why do I get such bad MPG while running the air (and not very high either)?
Is this something that I can fix?
Does it take more power from the engine to power the A/C thus more gas?
would a better air intake fix this issue? (yes, I know it would, but I can't afford it right now.)

I'm at a loss. Thus I appeal to all of you, and your collective expertise to help answer this question, and maybe help resolve this puzzle too.

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Old 02-25-2016, 06:48 PM   #2
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Different air intake won't have any effect on the MPG to make up for A/C operation.

Yes the A/C uses power from the engine to work, the fan level doesn't have a large effect on how much. Unless the system is broken (compressor not working, partially or completely "frozen" so hard to turn) the amount of load isn't bad for MPG except in low speed stop & go driving.

At a stop it uses appreciable fuel, as the engine has to run faster just to power it & stay running. When on the Hwy., the load is low enough to be BETTER than driving with the windows down. Less wind resistance from having them up is a larger effect than the power to run the A/C.

Gauge needle can happen to move at any time based on fuel used until then (not always smooth movement), Actual difference from running A/C needs to be tracked like any other MPG calculation. Run long enough using it in normal driving to go through most of a tank & check miles/vs. gallons. Remember each tank full will be a different result just from varied fill levels, so you need an average for good numbers. (5 mi of stop & go would be poor MPG anyways if that was the situation)

Not really a performance modification question, so I'll move this to General Tech Chat to go with other MPG threads.
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Old 02-25-2016, 07:02 PM   #3
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My needle can read over full for a good 45 minutes of highway driving then BAM! It's realistic.
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Old 02-25-2016, 09:14 PM   #4
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I agree with most of sailor's statements that an intake won't compensate for the losses and that the compressor seeing minimal use could make things turn less easy.

I disagree in that I have found on my Focus (and my old Saturn) that highway MPG with AC on is worse than windows down. My thought is that the better MPG with AC on is based off bigger engines that make more torque and have power to spare. Our little 4 cylinders need all the help they can get though and I can feel bring more sluggish with AC on.
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Old 02-25-2016, 09:49 PM   #5
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I was only mentioning a broken one BTW, minimal use though working shouldn't make a difference.

Won't argue Hwy MPG for sure, haven't put 300+ miles at a time on it windows down. Got a solid 38+ @ 67 and 36+ @ 70-75 with A/C on, good enough not to try hard for more. Got no worse with a smaller car years ago (after the new type compressors were in use, rotary draws less power than the old piston type) so I never tested seriously again.

Roll 'em down is certainly better for local driving, makes no sense to try & cool down a hot car with the A/C. Use it when it's really hot for longer drives after it cools down from the breezes.
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Old 02-26-2016, 08:08 AM   #6
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Gotcha, I must have misunderstood your post; sorry, sailor. You're doing way better in the MPG game than I am. If I could get 36 @70mph with the AC on, I'd be blasting it the whole way down the highway. But my car feels like it can barely climb bridges and gets low 30s whenever I use the air. Windows down and I'm able to get 36+. Used car with 190K on the odo now could be a culprit for me as well, not sure how well maintained the car was for the first 150K before I bought it and I haven't done anything to it yet other than change the oil.
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Old 02-26-2016, 04:08 PM   #7
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Extra 100k could make a difference, but my numbers are Max. in perfect Hwy. measuring conditions - all interstate travel.

If you get that good with the windows down, it's def. working for you. Buffeting gets worse for me with them much more than half way down, but the sunroof that came on it helps move more air.
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Old 02-26-2016, 06:47 PM   #8
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Shooot. In Memphis, 95+% humidity all through the summer thanks to that muddy river, blacktop asphalt everywhere, ambient temps at 100F, and temps more like 110F about 2' off the blacktop..... I roll my windows up every time. Well, after driving about a mile with the windows down and AC on to get the trapped heat out. You can't leave your windows down in Memphis.... if you value anything inside your car.
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Old 02-29-2016, 01:05 AM   #9
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That all makes sense to me, thanks you guys, I appreciate the input. I've always been curious, and now I know. I am however very jealous of your guys mpg, im lucky if I get 25 on the highway and 20 on the street.. Not sure how to make that better. Lol
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Old 02-29-2016, 10:10 AM   #10
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Don't forget, our duratecs are slightly better and my Hwy. numbers are for exclusively Hwy miles on long trips - the best possible scenario. NOT an "I drive mostly interstate to work", that ends up as a 'mixed driving' number.

Mid to high 20's is about it for a mix of driving in the local area, and not into the 30's unless traveling.
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