car A/C + efficiency

[daguuy]

I always drive my car at the lowest practical RPM (I drive a stick so I can do that) to use less fuel but like to use the AC when it's hot/humid (pretty much all sumer in STL). If I turn on the AC and put that extra load on the engine but still drive at the same low RPM that I always do, would it use more gas? Or does each engine revolution at a certain rpm always use the same amount of fuel, no matter what the load is (assuming I still have all the power I need to drive without down shifting)?

[hack124x768]

The load drives the RPM down, to compensate, you give it more gas. Its really that simple. If you have the AC on, you need to give it a bit more throttle to maintain the same RPM, which means more gas is being injected into the engine. From what I have heard though, it shouldn't make too much difference in your fuel efficiency.

[Limewater]

Hack is correct. Running the air conditioner does put an extra load on the engine and required more fuel.

Whether it affects your gas mileage a little or a lot depends on your vehicle. It makes a noticeable difference in my old 1991 Ford Escort, but your car is probably newer than mine. Newer vehicles apparently tend to have more efficient air conditioners. On my car, the A/C is the third brake after my main and parking brakes.

However, and I've never tested this myself, I've also heard that when driving on the highway you use less energy running your air conditioner than you do keeping your windows rolled down.

[vskid]

The throttle just sets how much fuel/air is being dumped into the cylinders, the RPM is just how fast the engine can go under the current load with the current throttle setting. Like if you slam the gas with the clutch fully engaged and the car is loaded down, you dump fuel into the engine, but it doesn't increase the RPMs as fast as if there was no load. Like our Celica we had, when you'd turn on the headlights while idling, the RPMs would drop a bit because of the added load but same air/fuel mixture. So if you have more stuff on, you're using more gas than if you had nothing on at the same RPM.

Mythbusters found that at lower speeds, windows down is better, but at freeway speeds, AC is better.

[bicostp]

The AC really kills my car (1999 Saturn, 90 HP 1.9L I4 engine), but I figured out a pretty easy way to cool the cabin down without hurting mileage too much. Only run the AC when you're not accelerating or doing more than 45 mph, and when it's off set the climate control to recirculate to keep the cool air in. Even with intermittent AC use I still get around 36 MPG on the cheap stuff. (Without using the AC at all I get about 38, sometimes 40 on a fresh oil change. I ran the AC all the time once and barely got 31. )

Those Ventshade things that go at the top of the window opening help too, since they let you keep the windows open a little without worrying about rain or stupid kids who will steal your radio, change, and CDs. It's a lot easier to cool a car when it's already the same temperature inside as the outside air.

Oh, and if you park in the sun a lot, get one of those foam reflective windshield curtains. They unfold and cover the windshield from the inside, keeping the sun from heating up the interior as much.

[Jongamer]

The throttle just sets how much fuel/air is being dumped into the cylinders, the RPM is just how fast the engine can go under the current load with the current throttle setting. Like if you slam the gas with the clutch fully engaged and the car is loaded down, you dump fuel into the engine, but it doesn't increase the RPMs as fast as if there was no load. Like our Celica we had, when you'd turn on the headlights while idling, the RPMs would drop a bit because of the added load but same air/fuel mixture. So if you have more stuff on, you're using more gas than if you had nothing on at the same RPM.

Mythbusters found that at lower speeds, windows down is better, but at freeway speeds, AC is better.

Yep..... I never saw that episode though, and car myths are my favorite ones, I only saw the fuel efficientcy one with the pick-up truck tail gate down is more fuel efficent (Which is busted) And tail gateing a 18 wheeler (Confirmed but not safe)
and those stupid attachments to your engine off the interwebz didn`t do crap.

I like the Egg in the Radiator episode the most.

[Dr. KillGood]

Myth busters did this, more efficient to drive with AC then windows down.

[nightwheel]

Here is my thoughts about this. Lately around here is be avargeing 100*F to 105*F. So im saying screw the gas efficiency and crank the AC up and enjoy.

[hack124x768]

Here is my thoughts about this. Lately around here is be avargeing 100*F to 105*F. So im saying screw the gas efficiency and crank the AC up and enjoy.

Amen!

Too bad I live in Seattle, It's not even 70F here these past couple days.

[Dr. KillGood]

Here is my thoughts about this. Lately around here is be avargeing 100*F to 105*F. So im saying screw the gas efficiency and crank the AC up and enjoy.

Amen!

Too bad I live in Seattle, It's not even 70F here these past couple days.

I HATE YOU!!!!

[nightwheel]

Here is my thoughts about this. Lately around here is be avargeing 100*F to 105*F. So im saying screw the gas efficiency and crank the AC up and enjoy.

Amen!

Too bad I live in Seattle, It's not even 70F here these past couple days.

I HATE YOU!!!!

Do you hate hack124x768 or do you hate the both of us?

[Krepticor]

I ahte all f you it has been triple digits here in SC and the humidity has been awful, try working in that and not suffocating