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Actual data: More on AC and slowing a car

Dear Tom and Ray,

Back on July 2, you told Amanda’s husband John that turning on the air conditioning was an ineffective way to slow down his car. You made up some nonsense about idle speed, which is clearly absurd. When the engine is braking it’s spinning faster than idle speed!

As you two appear impervious to the logic of the situation, my wife and I figured that some data might convince you. We have a 2007 Nissan Versa with a 6-speed manual transmission. There is a 300 yard stretch with a moderate slope on the way into town. For the last month, each time we’ve left home we stop at the beginning, turn the AC on or off, put the windows up, shift into 2nd, let the clutch up gently, and let the car go down the hill without touching the accelerator. The test is to see how fast the car is going at the end.

The results are clear. The 20 trials with air conditioning had a mean speed of 22.2±1.5 mph while the 20 without AC reached 26.3±2.4. Using a two-tailed T-test, we can dismiss the null hypothesis with >99% certainty and conclude that the air conditioner significantly aids engine braking, slowing the car about 4.1 mph. This is a larger effect than having surfboards on the roof racks, which cost 1.1 mph, or who drives. (She’s 1.3 mph faster.) Each additional person in the car adds 1.2 mph with no AC, but nothing with AC on.

You guys blew it! Of course whether this translates into a significant savings in gasoline, brake wear, or marital strife is beyond the scope of this experiment.

Steve in Honolulu

While that may be the case with your car, I can tell you for certain it is not with mine. My car is noticeably harder to slow to a stop with a/c on than with it off. Also it has to be held at a stop with the brakes when the a/c is on but not with it off.

If you are trying to reach Tom and Ray, you can’t get there from here.

Was Amanda’s husband also driving a 2007 Nissan Versa? If not, your “actual data” means nothing. In order to prove Tom and Ray wrong, you would need to test teh same model Amanda’s husband drives.

I can think of several cars I’ve driven that don’t slow a bit when the air conditioning is running.

the smaller the engine, the harder it has to work to keep the AC going

Actually, when people go to the e-mail page, in lieu of contacting Tom and Ray by that medium, they’re instead referred to post their comments to the guys to the discussion boards.

Amanda’s car is a 2004 VW Golf. Oddly, they never inquired whether it was a manual or automatic transmission. I assume it was a manual because the automatic won’t do much engine braking anyhow. Might be wrong.

John, Amanda’s husband, asked about slowing down from 60 to 40mph, when idle speed is plainly irrelevant. It sounds like DfromSD has an automatic and is thinking about very slow speeds, so idle speed does matter in that case.

While the numbers I got apply only to my car and the slope of road I was using, it does establish that the air conditioner can help slow a small manual transmission car that’s under 30 years old. That flatly contradicts Tom and Ray. This can probably be generalized to other manual transmission vehicles, though the relative importance of engine braking vs AC braking will vary.

Ideally, John would do some trials himself, perhaps taking a flat stretch of road and timing how long it takes to slow from 60 to 40 with and without the air conditioner. He could then get on Stump the Chumps and reveal Tom and Ray’s fallacious reasoning.

Your right Steve, my car is an a/t, and I too wondered about the difference between auto. & manual trannys when it came to that.

“Actually, when people go to the e-mail page, in lieu of contacting Tom and Ray by that medium, they’re instead referred to post their comments to the guys to the discussion boards.”

@cdaquila - In that case, as I’ve suggested before, the “Ask a Question” area of the page should have a short bit of text that explains to people exactly what they are doing. Something short & sweet like “This will post your question to an internet discussion board where other CarTalk listeners will read it & try to help answer and/or discuss your question”

That will keep people from doing completely ridiculous things such as posting “manifold leak” as the body of their text, or beginning their post as if its a “letter” to Tom & Ray. This is one thing that draws out some of the cheeky stuff of which the site admin doesn’t always approve. Its a simple thing. Its still baffling to me that it isn’t made clearer to people.

Manual vs Automatic transmission is the key factor. I can definetly feel the a/c cut in, slowing the vehicle, in my manual shift Honda Element, when running around 2500 rpm.