San Diego Union-Tribune (Sunday)
Improving car’s aerodynamics will mean better mileage
Dear Car Talk:
I removed the cross bars on my 2019 Toyota RAV4. My beloved queried, “What for?” I said to eliminate the jet propulsion din if we open the moonroof AND to get better mileage.
“How much better?” she asked.
So, now I need your help. Will I actually get better mileage, or am I full of bologna?
P.S.: I miss your brother and his laugh. My brother and I miss our younger brother, especially his laughter.
Thanks, Paul. I don’t know whether you’re full of bologna. But I’m sure your beloved has done extensive research on that, so ask her.
As for your car, improving your aerodynamics will absolutely improve your mileage.
Carmakers know that anything that “catches” air adds noise and reduces fuel economy. Basically, the harder your car has to work to push through the air, the more fuel it has to use.
So engineers and designers go to great pains and enormous expense, using computer models and wind tunnels, to make sure that air slips easily and smoothly over and around your car.
And then we — the buying public — take these carefully designed aerodynamic cars home and stick our kayaks on
— Paul
top of them.
I’m sure it makes the engineers cry when they pass us on the road, Paul.
Even without the kayak, the roof rack itself makes your car less efficient. One study, from Berkeley Labs and the National Renewable Energy Laboratory in 2015, using crowdsourced data, reported that the loss from a roof rack alone can be up to 25 percent. That sounds high to me.
Consumer Reports, which does extensive fuel economy testing, found that the loss from an empty roof rack was between 2 and 11 percent. The sedan they tested, a 2019 Nissan Altima, saw its highway mileage drop 11 percent when driven with an empty roof rack.
The other vehicle, which happens to be just like yours, Paul — a 2019 RAV4 — saw its mileage drop about 2 percent with a roof rack. Consumer Reports speculates that the RAV4, as an SUV, was already a boxier and less efficient shape than the sedan, so the impact of the roof rack was lower.
It’s like when I broke my nose — it didn’t make me look much uglier, given where I was starting from.
So the answer, for your car, is 2 percent. And the answer for everyone else out there driving 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 had problems with his 1947 Plymouth with a 6-volt electrical system. You suggested upgrading the electrical system, along with the wiper motor and other stuff, to 12-volts.
As I remember, my ’48 Plymouth had a vacuum-driven wiper. Can you explain? Thanks.
— Old Geezer Jim
Yes, that was just before they discovered the earth was round, right, Jim?
Actually, it was pretty common in the old days to have vacuum-powered wipers.
When the pistons descend in their cylinders, they create suction. That’s what sucks in the gasoline and air.
And some smart person figured out that you could use that suction to run things. In fact, there are still vacuum-powered parts in modern cars. Some vent doors in the heating and cooling system are operated by vacuum motors. And early cruise control was vacuum operated.
The problem, as you may remember, Jim, is that when you really open the throttle, the vacuum drops. Vacuum is high when the car is idling and low when you’re accelerating.
Think about your home vacuum cleaner hose. It’s a about an inch and a half in diameter, and you have plenty of vacuum. Now, imagine that hose is 3 feet in diameter. You’d feel very little suction, right? That’s essentially what happens when the throttle is wide open.
I’m sure you had the experience of stepping on the gas hard in your ’48 Plymouth and having your wipers stop. Not exactly the height of safety engineering.
In more recent times, they used vacuum reservoirs, which were just plastic reservoirs that would hold vacuum in reserve, to be used when the car was under hard acceleration.
So, at the time, the 6-volt electrical system was an an upgrade over vacuum wipers; 12 volts is even better. And now, we’re seeing cars with 48-volt systems. Maybe I should have recommended one of those to Jerry. He’d set the Guinness record for windshield wiper speed on a 1947 Plymouth.
Post questions online at Cartalk.com.