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Old 07-15-2019, 07:32 PM   #1 (permalink)
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Better mileage with AC on?

Had a near record tank going and then the wife says she wants to take the truck on date night. She likes her air conditioning and in this heat I wasn't risking making date night into an argument so AC it was.

I figured it was going to drop the MPG numbers pretty good but surprisingly, it didn't move them much at all. Hmmmm........

I know that windows down and AC off at highway speeds is supposedly the same as windows up and AC on at highway speeds, but I usually run windows cracked AC off/vents on at highway speeds, and have been getting stellar mileage. This tank the mileage wasn't that different.

So the question is this......

Was it just the fact that date night was only the last 90 miles of a 450 mile tank and I only paid a minor penalty, and it was really going to be a record tank?

Or...........did the extra drag on the engine from running the AC cause me to step into the gas pedal slightly harder (more engine load), negating some of the pumping losses from running with such a light foot? (I hope I have that concept right?)

I guess the real way to tell is drive to work in the cool mornings with the windows cracked like always, and then roll them up and run the AC every day on the way home and see how the tank compares. This week should be a good week for an experiment with temps in the 90's and heat indexes over 100 until this weekend.

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Old 07-15-2019, 09:37 PM   #2 (permalink)
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Several things probably. If it were a date NIGHT, then perhaps the drivetrain was well warmed from the daytime heat but the A/C didn't have to work as hard since it was in the evening? As well as the heat meaning less dense air so less drag and a bit of a WAI effect?
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Old 07-15-2019, 09:48 PM   #3 (permalink)
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Definitely a hot night. I think it was 80+ at 8pm and loaded with humidity.
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Old 07-15-2019, 10:55 PM   #4 (permalink)
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Quote:
Originally Posted by hat_man View Post
Was it just the fact that date night was only the last 90 miles of a 450 mile tank and I only paid a minor penalty, and it was really going to be a record tank?.
That would be my guess. I'd use A/C if I had it! No penalty if used while braking or going down steep hills!
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Old 07-16-2019, 08:03 AM   #5 (permalink)
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Quote:
Originally Posted by ksa8907 View Post
Several things probably. If it were a date NIGHT, then perhaps the drivetrain was well warmed from the daytime heat but the A/C didn't have to work as hard since it was in the evening? As well as the heat meaning less dense air so less drag and a bit of a WAI effect?

Bingo! A/C won't hurt mileage if it's not hot. The main point is if there's not sun out the A/C has an easy time - this effect is easily observed if you pull into a carport or it becomes overcast on a hot day.

On an old vehicle like an '03, the A/C penalty is potentially huge, I actually ran out of fuel once because I refused to believe I had used so much fuel in such a short distance - it was a 110* week and the A/C killed my range.

After that I just mostly sweated it out, using A/C only on DFCO.

In my 2016, you'd be hard pressed to tell much difference, modern computers run the A/C during deceleration as much as possible (plus I still do it manually), variable geometry pumps etc means that my best tanks are often with the A/C on quite a bit.
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Old 07-19-2019, 01:48 PM   #6 (permalink)
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Modern AC is certainly more efficient than it used to be, but to put it in perspective, my i3 can use up to 2.5kW at full AC load. In my car this works out to additional drag close to driving at 25 mph - or about 16% of total drag at 65 mph.

I open the windows when I can around town, but at highway speeds I don't think I am giving up much with the AC on particularly since it is rarely needed at 100%.
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Old 07-20-2019, 09:19 AM   #7 (permalink)
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I've seen double the fuel use than what I'd expect on similar stretches, when using AC on full tilt in very hot weather
That's city driving , ie lots of stopping, accelerating, ... and a small car / small engine where you can really feel the difference when accelerating with or without AC.

Out on the motorway or open road, the difference is far smaller and vanishes in normal variance.
With a 2,3L and thus 2.3 x bigger engine, you should see little effect


On average, AC cost 4% more fuel for me
0.15 kg/100km extra on 3,67 kg/100km overall average
(0.15 rather than 0.12 as the average also includes 1/6th mileage with AC on)

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Last edited by euromodder; 07-21-2019 at 09:39 AM..
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Old 07-20-2019, 10:27 AM   #8 (permalink)
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1). It is harder to move the average once established over many miles.

2). Easing along near dark with A/C on isn’t the penalty it would have been at mid-afternoon.

3). Given a vehicle well-warmed up that day (45+ miles), THAT particular penalty is in abeyance.

Having window covers (interior) is a real help. I’ve made quite a few from 4’ tall rolls of REFLECTIX. As with a cold engine needing time/distance to warm up, ANYTHING that reduces heat load into the interior pays for itself.

Strictly speaking, no, there won’t be “better” fuel mileage with HVAC use. Reducing the penalty is the game.

.

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