Dear Car Talk: Your column is always mandatory reading for me, and I look for it first thing. I own a 2016 Nissan Murano with 69,000 miles on it. It is serving my family well. My question is: What is up with its aerodynamics? I live in Phoenix, and it's very dusty in the desert here.

The rear window and lift gate get dusty after driving just a few miles down the road. Does that happen with other, similarly shaped cars? — JR

Answer: Yup. Believe it or not, there are physicists who spend their entire careers studying stuff like the flow of water and dirt onto car surfaces, JR. Who knew the car wash industry had so much influence over our nation's research agenda?

Anyway, the answer is: yes. "SUVshaped" vehicles are prone to having dirty tailgates and rear windows. Here's why: The primary goal of aerodynamics on passenger vehicles is to let them slip through the air, rather than fight it.

If you put a billboard for Juicy Fruit gum on your roof, the air would crash into it and make it harder for the car to move forward, reducing your fuel economy. The same is true for smaller items. The rake of the windshield, the shape of the side mirrors and the arrangement of parts underneath the car can all affect aerodynamics.

That all makes sense, right? Well, because of the shape of SUVs, the air flowing quickly over and under the car creates a vortex when it comes out the back. And that vortex, in effect, sucks up the dust and dirt being kicked up from the road and distributes it all over the back of your freshly washed SUV.

A sedan, with a more bullet-shaped profile, creates a much smaller vortex behind it. It also has less flat surface area (and no nearby window) back there for that vortex to deposit dirt on.

The solution? A rear wiper. Or, if that's not acceptable, you can fashion a giant rocket-cone that fits on the back of your Murano to make it more bullet-shaped. Then you'll be all set for the next Burning Man, too, JR.

Write to Ray in care of King Features, 628 Virginia Drive, Orlando, FL 32803, or email by visiting www.cartalk.com.