AC question!!! Which saves more gas?!?!

DaveyG · 08-25-2011, 08:12 AM

Quote:

Originally Posted by wstar

A related issue that's been debunked before: rolling down the window vs running the A/C. Assuming you can bear the heat level of running with the windows down and no A/C to save gas, it actually does save gas at low speeds (city streets). However, on the highway, the parasitic drag on the car from having the windows open wastes more gas the running the A/C, so don't do that.

So true, I wish people would understand this. I swear I always run into people saying ac uses more gas no matter what. Either you understand physics or you don't I guess.

Red__Zed · 08-25-2011, 08:52 AM

Quote:

Originally Posted by DaveyG

So true, I wish people would understand this. I swear I always run into people saying ac uses more gas no matter what. Either you understand physics or you don't I guess.

It's more of a fluids question...

...but at 70 or so, most cars use about 2.5% more gas with the windows down, and 5% more with AC on, so.....

wstar · 08-25-2011, 09:17 AM

Mythbusters tested this (measuring fuel consumption and efficiency in real cars), and then went back and revisited the issue after comments and concerns and re-tested. Wikipedia's final summary of their final summary:

The fundamental flaw in the MythBusters' test was that the point where the drag becomes powerful enough to inhibit a car's performance with windows down was inside their 45–55mph margin at 50mph. Going less than 50mph it is more efficient to leave the windows down, but going greater than 50mph it is more efficient to use the A/C.

ETA: It's probably a reasonable assumption that the 50mph figure will vary depending on the aerodynamic configuration of the car and the windows, as well as other differences affecting the A/C compressor's efficiency. But it's a reasonable ballpark, and reasonable cars' break-even point probably doesn't vary a whole lot.

08-25-2011, 09:17 AM	#3 (permalink)
wstar A True Z Fanatic Join Date: Mar 2009 Location: Houston, TX Posts: 4,024 Drives: too slow Rep Power: 3596	Mythbusters tested this (measuring fuel consumption and efficiency in real cars), and then went back and revisited the issue after comments and concerns and re-tested. Wikipedia's final summary of their final summary: The fundamental flaw in the MythBusters' test was that the point where the drag becomes powerful enough to inhibit a car's performance with windows down was inside their 45–55mph margin at 50mph. Going less than 50mph it is more efficient to leave the windows down, but going greater than 50mph it is more efficient to use the A/C. ETA: It's probably a reasonable assumption that the 50mph figure will vary depending on the aerodynamic configuration of the car and the windows, as well as other differences affecting the A/C compressor's efficiency. But it's a reasonable ballpark, and reasonable cars' break-even point probably doesn't vary a whole lot. __________________ 7AT Track Car! Journal thread / Car setup details

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