With windows up: affect of A/C on mileage?

[Sally_C]

I had a discussion with my husband about how much does running the A/C affect mpg. He has a '13 Mazda 3 and I have a '14 CX-5. When the windows are down, we've read that it depends on how fast you're going - drag & resistance sort of factors - which isn't really what we were wondering about. We're curious about when the windows are up. Has anyone played around with that scenario and do you notice any difference then? He's also thinking how much the size of your engine/car made too. So it might not matter much in the CX-5 but make more of a difference in the Mazda 3. He turns his on/off based on humidity and heat - which makes sense, I just tend to turn it on and forget about it even in the winter. But now I'm wondering if I should pay more attention to it.

Curious what some of you have experienced or think?

 

[Chris_Top_Her]

I lose up to 4 mpg when hot humid
. I also drive 70-85.

 

[erhayes]

My CX7 lost ~ 1 mpg with the AC at 100% in local driving.

 

[Nodnerb]

Well, the compressor puts a load on the engine so it will always use a bit more gas with the air on, or even the front defrost. You will also even lose a tiny bit of power.

 

[robertmazda]

Agree with Noderb. If the windows are up either way, using the A/C will of course put additional load on the engine and reduce mileage. In the overall scheme of things though, I doubt that it will make that much difference.

 

[Buger]

I had a discussion with my husband about how much does running the A/C affect mpg. He has a '13 Mazda 3 and I have a '14 CX-5. When the windows are down, we've read that it depends on how fast you're going - drag & resistance sort of factors - which isn't really what we were wondering about. We're curious about when the windows are up. Has anyone played around with that scenario and do you notice any difference then? He's also thinking how much the size of your engine/car made too. So it might not matter much in the CX-5 but make more of a difference in the Mazda 3. He turns his on/off based on humidity and heat - which makes sense, I just tend to turn it on and forget about it even in the winter. But now I'm wondering if I should pay more attention to it.

Curious what some of you have experienced or think?

At least at idle, it is easy to measure by turning the AC on and off while measuring the instant gas usage with a tool or app that can read the CPU data and seeing the difference. I've previously noted the below gas usage at idle:

At idle, my CX-5 uses around .12 to .15 gallons per hour when fully warmed up. At initial warmup, it uses a tremendous amount of gas (about 2 gallons per hour at idle) and retards ignition timing to about -25 degrees until the cat temp hits about 350 F, then immediately drops down to .5 gallons per hour and slowly drops after that.

Off the top of my head, I think the AC used about a tenth of a gallon more per hour when idling. If this remains constant over different speeds and different conditions (like acceleration) then maybe the impact could be around 10%?

As an example, if you are driving 55 on the freeway and getting 30+ mpg, 10% more gas could be about 3mpg less.

 

[GAXIBM]

Buger - yes but driving at 55 the compressor will cycle and actually use less power as a % of total engine load as the evaporator cools to freezing to avoid freezing up. At idle the electric radiator fans must run to cool the engine and the AC while at 55 they have airflow and will cycle off reducing a sizeable electric load. The compressor must run a lot more as a % of the time at idle as it is turning at a much lower rate and it is harder to get to the freezing temp. Point is, 10% at idle not equal to 10% at speed.

 

[Buger]

Buger - yes but driving at 55 the compressor will cycle and actually use less power as a % of total engine load as the evaporator cools to freezing to avoid freezing up. At idle the electric radiator fans must run to cool the engine and the AC while at 55 they have airflow and will cycle off reducing a sizeable electric load. The compressor must run a lot more as a % of the time at idle as it is turning at a much lower rate and it is harder to get to the freezing temp. Point is, 10% at idle not equal to 10% at speed.

I figure a/c usage of gas would likely be very different depending on different speeds and conditions (acceleration, cruising at speed, deceleration of course, etc) which is why I bolded the assumption about how the approx 10% figure would only be significant if it remained constant. Knowing exactly how much extra gas is used at idle is only a very small part of the picture.

 

[erhayes]

A long time ago, General Motors stated that the cost of running their AC was less than running with windows down, at highway speeds.

 

[Sally_C]

You guys are great! I am enjoying reading the comments and explanations. I've been fiddling with this myself - in a much less scientific way - and it doesn't appear to me that running the A/C makes much difference. Less than 10%, if that. It seems like which way the wind is blowing, and how hard, has more affect. But I'm not mechanically inclined so my "testing" is more like running a trip with it on, then running the same trip with it off, and comparing the difference. I really am finding your comments interesting - thanks!

 

[GAXIBM]

I also think that when it is hot and you run the AC the air is thinner and it takes less energy to move the car so when hot and running AC the mileage does not go down. Also with hot air outside the temp sensors and injection profile are in a different area perhaps yielding a higher air/fuel ratio and more efficient output(?) My mileage on my 2000 Corvette seems to go up the hotter it gets - even though I think I am using the same summer blend fuel in both cases... I have always read that cooler air make a more efficient gas engine but my experience with the total effect in a auto does not yield this result...

 

[gova]

This comes to mind.

with windows down CX (no pun intended) goes up dramatically.