Improving the car’s aerodynamics will mean better mileage

Dear Car Talk:

I removed the cross bars on my 2019 Toyota RAV4. My bestie asked, “What for?” I said to eliminate the jet drive noise if we opened the sunroof AND get better mileage.

“How much better?” she asked.

So now I need your help. Am I actually going to get better mileage or am I full of bologna?

PS: I miss your brother and his laugh. My brother and I miss our little brother, especially his laugh.

— Paul

Thanks Paul. I don’t know if you’re full of bologna. But I’m sure your sweetheart has done extensive research on this, so ask her.

As for your car, improving your aerodynamics will absolutely improve your mileage.

Car manufacturers know that anything that “catch” air adds noise and reduces fuel consumption. Basically, the harder your car has to work to push air through, the more fuel it has to use.

So engineers and designers go to great lengths and great expense using computer models and wind tunnels to make sure the air slides easily and smoothly over and around your car.

And then we – the buyers – take these carefully designed aerodynamic cars home and stick our kayaks on them.

I’m sure it makes the engineers cry when they pass us on the road, Paul.

Even without the kayak, the roof rack alone makes your car less efficient. A study conducted by Berkeley Labs and the National Renewable Energy Laboratory in 2015, using crowdsourced data, reported that the loss from a roof rack alone could be as high as 25 percent. That sounds high to me.

Consumer Reports, which conducts extensive fuel economy testing, found the loss from an empty roof rack to be between 2 and 11 percent. The sedan they tested, the 2019 Nissan Altima, saw its highway mileage drop by 11 percent when driven with an empty roof rack.

The other vehicle that happens to be exactly like yours, Paul—a 2019 RAV4—saw a mileage drop of about 2 percent with a roof rack. Consumer Reports speculates that the RAV4, as an SUV, already has a boxier and less efficient shape than the sedan, so the impact of the roof rack is less.

It’s like when I broke my nose – it didn’t make me much uglier considering where I started.

So the answer for your car is 2 percent. And the answer for everyone else who drives with a roof rack or removable cross bars is to take them off when you don’t need them. Or rent a kayak when you get there.

Dear Car Talk:
In a recent column, you answered a question from a guy named Jerry who was having trouble with his 1947 Plymouth with a 6-volt electrical system. You suggested upgrading the electrical system, along with the wiper motor and stuff, to 12 volts.

As I recall my ’48 Plymouth had a vacuum powered wiper. can you explain Thanks.

“Old Geezer Jim.”

Yes, this was just before they discovered the earth was round, wasn’t it, Jim?

In fact, in the old days it was quite common to have vacuum powered wipers.

As the pistons descend into their cylinders, they create suction. This is what sucks in the gas and air.

And some clever person figured out that you can use that suction to drive things. In fact, there are still vacuum driven parts in modern cars. Some ventilation doors in the heating and cooling system are operated by vacuum motors. And the early cruise control worked with a vacuum.

The problem, as you may recall Jim, is that when you really open the throttle, the vacuum drops. The vacuum is high when the car is idling and low when you accelerate.

Think of the hose on your home vacuum cleaner. It’s about an inch and a half in diameter and you have plenty of vacuum. Now imagine that this hose is 3 feet in diameter. You will feel very little suction, right? Essentially this is what happens when the throttle is wide open.

I’m sure you’ve had the experience of stepping hard on the gas in your ’48 Plymouth and having your wipers stop. Not exactly the height of the safety device.

In more recent times they used vacuum tanks which were just plastic tanks that held vacuum in reserve to be used when the car was under hard acceleration.

So, at the time, the 6-volt electrical system was an upgrade over vacuum cleaners; 12 volts is even better. And now we see cars with 48-volt systems. Maybe I should have recommended one of them to Jerry. He had held the Guinness World Record for the speed of the Plymouth sweepers since 1947.

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(c) 2023 by Ray Magliosi and Doug Berman
Distributed by King Features Syndicate Inc.

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