4 Gas Saving Myths

[boulder_bum]

I saw an interesting article on CNN.com:

http://money.cnn.com/2007/05/09/pf/gas_myt...dex.htm?cnn=yes

Among other things, they claim that having the windows rolled down or using A/C doesn't really affect your mileage.

Most of the stuff is pretty believable, but I'm suspicious about the A/C claim. Do any of you have any other hard data on air conditioning and Prius mileage?

[tripp]

I've heard something like a 7% ding if running MAX COLD pretty much full out. I don't use AC much, I kinda like a breeze blowing through the car (there's usually a lot of hockey gear in the back ).

[hampdenwireless]

The answer to the AC/windows down question depends on the car and the speed.

For most cars at highway speeds its best to have the AC on and the windows closed. As speed drops or there is starting and stopping its better to have windows open. The fan on high no ac is always the best for mpg.

[qbee42]

<div class='quotetop'>QUOTE(tripp @ May 9 2007, 09:37 PM) [snapback]438732[/snapback]</div>

I've heard something like a 7% ding if running MAX COLD pretty much full out. I don't use AC much, I kinda like a breeze blowing through the car (there's usually a lot of hockey gear in the back ).
[/b]

If you roll up the windows with hockey gear in the back, you'll die from the smell. That's why we normally play hockey in the winter, so all of the microbes are asleep.

Tom

[Delta Flyer]

A/C will make a much bigger fuel economy hit in stop-and-go traffic - not as bad while cruising.

Wonder if anyone else here cuts A/C off on acceleration - on while braking?

Shutting off the engine at intersections: a Canadian website says if you idle more than 7 seconds, it's more gas than by restarting. Idleing is considered "severe engine wear" - gunk builds up in the engine, so make sure autostop is on as much as possible.

[Devil's Advocate]

The Mythbusters tackled this one and got conflicting results. Mostly because the first round of testing was done at 45 mph but due to safety concerns for the second round they lowered the speeds to 35 mph.

In the first round the A/C windows rolled up SUV faired better than windows down,
In the second test the complete opposite.

I figure the transition point is about 40 mph

[richard schumacher]

There's one factor special to the Prius. If the outside air is uncomfortably warm it's better to leave the windows up and run the AC. The traction battery and its electronics are cooled using cabin air (the vent to the right of the rear seat). If that system gets too hot it runs less efficiently, and fuel economy will suffer.

[cnschult]

I would turn off your a/c in the summer at stop lights in order for the engine to stop, same thing in the winter, turn off both the rear & front defrosters so the engine will stop. idling uses fuel & causes wear & tear.

last night on history channel's modern marvels they talked about truck stops. more & more truck stops are being outfitted with a special HVAC duct that goes into a semi's window when the driver is resting/sleeping. It brings electricity for a tv/laptop, internet & cable access, brings in fresh comfy air and takes out the old air.

the show pointed out that new regulations require 10 hrs of rest for every 11 hrs of driving. that's a lot of idling time for a semi, the cost of the HVAC/electricity is cheaper than the fuel the truck will use, and as far as wear & tear, 1 hour of idling = 6 miles of driving. so the wear & tear savings and benefits to the environment are just a bonus on top of the money saved on fuel.

so the moral of the story is don't be afraid to run your HVAC while you are accelerating, just try to turn everything off but the radio when you will be stopped for over 10 seconds.

& don't drive the prius on the highway with the windows down, if you want open air "green" driving, put down a deposit on the Tesla.

[efusco]

Oye! Not again....

Condition 1) A/C on, windows up--uses energy to run A/C and therefore must impact FE...there's nowhere else to get the energy to run the AC

Condition 2) Windows Down, A/C off--increases drag, to make up the drag you have to use more energy...that comes from gas...lower FE.

Condition 3) Windows up, AC off--No drag, no A/C energy loss and thus no impact on fuel economy....you just die from heat stroke.

The thing that happens, like in the CNN article, is that it's been shown that there's little difference in the FE impact b/w windows down and A/C use...so they draw the false conclusion that using A/C or putting the windows down doesn't affect FE...they always fail to take into account condition 3 above...windows up/no A/C.

So, given the 3 choices above, hell ya I'm gonna use A/C most of the time when it's really hot out...but that doesn't mean it's not impacting my FE. Further, in the Prius, since the HV battery IS cooled by the cabin air running the AC can have additional benefits on FE...or lower the negative impacts anyway...by keeping the HV battery cool which makes it more efficient.

[slair]

I say just do what makes you comfortable. You will save what, 50 cents per tank if you keep the windows up and AC off on a mid summer day when its 110 in the car? I'd rather spend the 50 cents to be comfortable while I'm on a 4 hr trip. We're already owning everyone else when it comes to MPG.