Can Car Talk Solve Marital Dispute About the AC?

Dear Car Talk:

I'm hoping you can settle a family dispute. You see, my wife and I have come a long way since our college days at UC Davis, when we drove a 1967 Mercury Cougar. It was a great car, especially since gas was 75 cents a gallon, except it would overheat on every grade. Driving down to Los Angeles, we would run the heater going up the grapevine on 100-degree days to keep the temperature gauge out of the red.

My wife now drives a 2014 Lexus RX 450H, and I drive a 2014 4x4 Toyota Tacoma. As we make the climb up the Sierras to our vacation house, I always turn off the AC, partly because of all the old cars I had as a kid, but additionally because it is less work on the engine and cooling system to have the AC off. My wife says modern cars are designed to handle the load, and with her hot flashes, she does not want her thermostat screwed up any more by me turning off the AC.

I promise I am not turning on the heater anymore, although at times she thinks I am. So, can I leave the AC on going up a, let's say, 6 percent grade and make my wife happy, or should I continue to turn it off? These hot flashes are killing me.

-- Bill

Bill, this is a no-brainer. New wives are much more expensive than new engines. My late brother learned that a few times. 

But you won't even need a new engine. Your wife is right that modern cars are designed to handle the demands of air conditioning under all but the most extreme conditions. And since your wife drives a hybrid (the Lexus 450H), the air conditioner is not even run by the engine; it's run by a separate electric motor. So running the AC has no effect on that engine whatsoever.

And when you're driving your Tacoma, it has a temperature gauge on the dash. So if it's 110 degrees out, and you're trying to maintain 65 mph up a 6 percent grade, and you see the temperature needle moving up near the hot zone, then you can slow down and turn off the AC until you reach the top of the hill.

But most modern cars shut off the AC anyway under those "wide-open-throttle" conditions, when they need to divert maximum power to the engine. 

And, incidentally, slowing down from 65 to 55 or 50 mph will do a lot more to help cool the engine than turning off the AC will. 

So I'd suggest modifying your speed as a first step, before turning off the AC and ticking off your wife. Remember, even when it's so hot out, you could have a very chilly weekend up there in the Sierras, Bill.