AC on the entire time and I got my best MPG average ever??
 

As you all probably know, southern California is having lots of heat. I found myself using my AC more and more last week. Normally, I will drive home with my windows up, with no AC or the fan. Like many, I’ve been under the impression that driving with the AC on will drop for MPG, however, this last week, with my AC running the entire 40.5 mile drive home, I’ve averaged by best MPG ever with my same driving habits. Can anyone explain why my MPG did not decrease with the AC blowing?

Thanks

That just means your car is capable of that much better mileage, seriously.

Hello,

What it could indicate is that your A/C is more efficient than running with your windows down? Or, that your driving technique is much better now, and your mileage could be even higher if you could run w/o A/C?

My explanation is that you need to do multiple tests, not knowing how you calculated your MPG.

The AC requires energy input so it cannot increase your mileage. It has to decrease it, all other conditions being the same.

It is very common to get different mileage on two different tank fills even though you think conditions and your driving was the same on each. The fact is, conditions can vary considerably due to a large number of factors beyond our control.

I've found over the past few years that if I use air conditioning on the lowest setting, which is plenty cold, the effect it has on mileage is negligible.

regor -

Maybe it's the effect of the hot dry temps. Now that summer is in full swing, these are the best times for MPG.

It is possible that *without* AC, you would have seen even higher MPG.

I just recorded my all-time best "go home" commute segment last Friday. I attribute it to Hot Dry weather and slower average speed.

CarloSW2

MPG goes up with temps. Apparently yours went up enough to offset the small amount used by running the AC. Maybe you were also driving more? There are many things that effect mileage, as I have learned watching my ScanGauge II.
If you can try and get one of those, you will see!

slurp812 -

That reminds me. Whenever I have a "best run" for one of my trip logs, I save the weather from weather.com. I usually average the weather at the begin/end ZIP codes (a compromise, but I don't have on-board weather station, YET ;) ). This helps me to compare MPG runs better.

CarloSW2

Ive wondered about that sometimes. Im sure mine has more mpg available. Reason is I get 32 mpg highway no matter if Ive got the AC on or not, windows down or not, day or night. I just joined up here recently but Ive been driving this same trip for nearly 5 years. I also took a trip to Washington DC a couple years ago and with no stops between tanks I still got 32. I dont exceed the 70 mph freeway speed limit, thats plenty fast enough, I usually go 65-70. The other 70 miles at 55 does not improve mpg. On top of that, my car is not exactly the most aerodynamic thing. Im thinking Ive got enough torque where these things dont make much of a difference and therefore I would benefit from a final drive change.

Regor,
I noticed something similar to you this tank as well. Maybe the air temp.
I had "gratuitous" use of the AC on my return trips on my commute this tank because of the heat here in central california. I was able to break the 50MPG ceiling, I believe, because I was stuck behind a big rig for 30+ miles and was able to get a crazy increase in mpg on my ScanGauge2- somewhere in the mid 80's mpg range! I shot up my overall economy without even trying. Maybe you had a similar situation (of drafting).
The experience really does illustrate how much air resistance robs us of MPG. I never would've believed drafting would yield such an increase in mpg, if I hadn't seen it myself. It's almost like driving downhill! All the while the AC was on- so you can bet I'll be looking for more trucks on my commute.
I have also noticed a bit better MPG driving home in the hotter afternoon, than in the mornings, which lends to the warm air increase hypothesis.

Could you tell if your engine was running any hotter? Typically A/C condensers are mounted in front of the engine radiator and therefore your radiator is drawing pre-warmed air to cool the engine coolant. Combine that with the already high outside temperatures your engine may have been running hotter.

Engines that run hotter are theoretically more efficient because they come closer to approximating the Otto cycle ideal adiabatic process where no heat is lost during compression of the intake charge. Hotter cylinder wall/head temperatures mean less compression and combustion heat is lost to the coolant and more is converted to work. Modern vehicles are tuned around 205 degree thermostats because this temperature has been found to be more efficient than the 160-180 'stats commonplace 20 years ago. You only run into problems with higher heat if detonation occurs (especially if your ECU pulls back timing advance due to knock sensors avoiding knock conditions) then mileage decreases and damage can occur.

Humidity (relative humidity) can have a large effect on engine performance, possibly as much or more than temperature alone. My truck seems to like cooler temps and higher humidity so it's probably pulling in some timing to avoid knock on 87 in hotter/dryer conditions.

hey there I am in SD too and yeah its been HOT, I was stuck in more traffic than normal and using the AC the whole time. I kept thinking this tank is going to suck but then I filled up and still got 40!

has to be the hot dry weather:) or I am just gettin good with the hypermiling