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