Monday, May 7, 2012

Aerodynamics and car mpg

Aerodynamics makes little difference in auto and truck fuel efficiency until you reach 40 mile per hour air speed.  (40 into a 20 head wind is 60, 70 into a wind of 30 is 100)  Above 40 the cross section of the vehicle pushing through the air takes more and more energy the faster you go, and the increase grows at a faster rate than your speed grows, pissing away money the faster you go.  Windows down increase drag, even a dirty car increases it a tiny amount.
Did you ever think of the bottom of the car's effect on aerodynamics?  The bottom of a car or truck is a twisted mess of pockets and baffles creating significant turbulence and drag beneath it.
I crawled under my Hybrid Toyota Camry this morning, I get from 37 mpg on a tank and sometimes up to 41.  I was wondering what I would find.  Wow.  From the sides of the car is a smooth surface extending almost into the center.  Below the engine and a strip down the center to near the rear axle it's open, from there almost to the rear bumper is another solid surface all the way across.  Toyota has eliminated most the drag from the bottom of the car.   These panels are some kind of fiber material held on with a few bolts.
I don't know what the GOP and some of the American car and truck makers are bitching about on the new MPG standards.  Their are a lot of ways to improve without expensive technology; retract the windshield wipers out of the air stream, do a better job with side mirrors, low rolling resistant tires, give the car a sleeker shape and smooth fit, reduce the giant wheel wells on trucks and SUV's, flat hub caps, and put a smooth plate under the car.  Without touching the weight or the engine, a good gain is already possible.
A study in Europe found a flat wheel cover/hub cap on trucks (most are open or have a knot of bolts and nuts sticking out like evil chariot wheel), tested in wind tunnel and on road found one half a percent increase in mpg on trucks.  Thats a good bit of change off the 18 wheelers $73,000 yearly fuel bill, or around $365.  More than pay for the covers the first year.   If a guy had a fleet of trucks, it seems like a crime not to add wheel covers.   I recently saw a few trucks in Wyoming and Montana with flat wheel covers, surprised I don't see more.


  1. Isn't that why the semi's started having that curved thing on the roof - drag?


    1. yep, that gets some of the wind over the trailer, and now some of trailers have a side shield on them, that guides the wind along the side of the trailer and helps a little.


Anonymous comments might end up in the trash.