4 Gas Saving Myths

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 ).

 

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.

 

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

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

 

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.

 

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

 

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.

 

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.

 

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.

 

Being very aerodynamic, the Prius makes it hard to test, but the -anti-Prius-, a 2001 Nissan Pathfinder, told me in the owners manual, like the Prius owners manual, that windows up and A/C on during highway driving is more efficient. So I tested it. Long descent from a plateau (east end of hwy 24 in BC, descending to Little Fort).
Windows up, A/C on, the Pathfinder and Prius would reach a top speed higher than I was willing to go - I chickened out at 130 km/hr.
Windows down, A/C off, the Pathfinder topped out at 110 km/hr, the Prius 95 km/hr (presumably due to lower mass).

The Mythbusters tried to measure fuel burned, and we all know how difficult that is to do with accuracy! I measured drag, which is easy to do. The highway I used has little traffic, even in summer. It is also not heavily patrolled, which was more important as the limit was 80 km/hr.

I don't recommend this, as trucks descend this hill at 50-70 km/hr, and you can come up on them dangerously fast around a hairpin!! I was very careful (and lucky).

 

The Prius MPG Simulator predicts the following at 80F and 55MPH:

• No AC, windows up: 61 MPG
• No AC, all windows down: 53 MPG
• No AC, two windows half down: 57 MPG
• AC set to 75F windows up: 59 MPG

At 90F and 55 MPH:

• No AC, windows up: 64 MPG
• No AC, all windows down: 55 MPG
• No AC, two windows half down: 60 MPG
• AC set to 75F, windows up: 60 MPG

These all assume steady speed, level terrain, no wind, smooth dry pavement, Goodyear Integritys inflated to 44/42, and 300# load (cargo + driver).

 

Ok, correct me if I am wrong, but I thought the compressor for the A/C was driven by an electric motor, not a belt and pulley from the gas engine. Therefore, you will not degrade engine horsepower with the A/C, but you could divert electrical current. I doubt that you could divert enough electrical current for the A/C compressor motor to notice reduced horsepower output of the Synergy electric drive motor. I'll have to try that tonight on my way home...

 

Sounds right to me, but what does that have to do with the question being discussed on this thread? I thought the question was about fuel savings, not HP reduction.

 

The economics may be a bit different than you suggest, but I still think you should go for the comfort. I like going without A/C, unless the sun is beating down on the car (and me!), but it is a nice option when necessary.

Let's look at your 4 hour tour. If your mileage normally is 55mpg, then for your 4 hour drive of 250 miles you'll burn about 4.55 gallons of gas. At $4.00/gallon (national average, non?) you'll spend $18.20 for the burn. If A/C drops you to 51mpg (about a 7% drop), then you'll use 4.90 gallons at $19.60. Can you handle the $1.40 for the afternoon?

It may burst the $.50 bubble to tell you this, but for a tankful of gas you'll be spending well in excess of $3.00 just for your A/C. Actually, it comes close to a gallon of gas per tank (makes sense with a 7% drop).

Of course, this is only on highway use. If you do this around town, you might really start to burn money!

 

considering we burn a tank every two weeks or so... I don't care! I'll use the AC if it's uncomfortable. We're not Neanderthals afterall.. god gave us freon for a reason!! (jk - no need to respond fellas)

 

ACtually there's no Freon in your Prius.

 

I would have bet the bank that someone would post this....

 

Except those folks driving EVs or riding bicycles. But yeah, I get your point.

 

It has to do with another poster mentioning turning the AC on when braking, and off when accelerating. On a belt-pulley system, that makes some logical sense. In the Prius (or in EVs) it makes none.