July 17, 2008

Vehicle Speed and MPG's

I knew that your fuel economy dropped as your speed increases - the primary factor being aerodynamics - but I didn't think it was this much:
Today, 35 m.p.h. is no longer the best speed for autos with their sleek designs and advanced transmissions. Newer vehicles generally get the highest gas mileage somewhere between 45 and 55 m.p.h., says David L. Greene of the National Transportation Research Center at Oak Ridge National Laboratory in Knoxville, Tenn.

The main force reducing mileage is air drag, says Dr. Greene. The faster you go, the greater the drag. Drag forces increase exponentially, so doubling your speed from 40 to 80 increases drag fourfold.

It makes a huge difference, for at 80 m.p.h. your car pushes against wind with the force of a hurricane.

Consumer Reports tested the effect of higher speeds on gas mileage. David Champion, director of auto testing, found that boosting the highway speed of a 2006 Toyota Camry cut gasoline mileage dramatically:

•55 m.p.h. – 40.3 miles per gallon

•65 m.p.h. – 34.9 miles per gallon

•75 m.p.h. – 29.8 miles per gallon

On a hypothetical 1,900-mile round trip from New York City to Disney World in Florida, the Camry would use 47 gallons of gas at 55 m.p.h.. But at 75 m.p.h., it would burn nearly 64 gallons – a $70 difference.
My guess is that the differences would be even more significant for older cars.  I think this is the referenced Consumer Reports article.

PS - Not that the President would ask you to make such a "partiotic" decision as to save gas.


Anonymous said...

The mileage difference between 55 and 65 really surprises me. I've been driving 60 on the highway, and sensing a lot of bad vibes from the speed demons flying by. It may not be safe to slow down to 55.

j said...

I was surprised at that jump as well. Agreed on the dangers of going 55 - it really feels like you are a sitting duck - not to mention that it is actually below the posted minimum speed on some freeways.

Adam Rosien said...

I remember somewhat from physics class that drag increases as the cube of the velocity, not exponentially.

Chris O'Byrne said...

In response to anonymous, I've been driving 50 to 55 on the interstates and have had a few people (mostly truckers) honk their horns, but nobody has done anything foolish.

In response to j, I have never seen a minimum speed limit greater than 45 mph on the interstates.

And lastly, in response to adam, the drag increases as the square, but the power required to overcome that drag increases as the cube. That means that if you double your speed, it will take 8 times as much power to overcome that increase in drag. Crazy!

j said...

Chris -

thanks for dropping the physics knowledge. the only place i've seen the minimum speeds that high are the freeways around the Los Angeles area.

Chris O'Byrne said...

j, you're welcome and thanks for the info about LA... now wonder I try to avoid it! Last week I drove a 33-foot motorhome from Minnesota to Las Cruces and stayed between 35 and 40 the whole way. I left the hazard lights on and stayed off the interstate highways and everyone was so nice to me! Oh, I drove that slow because I'm a wuss when it comes to driving something that big that the wind blows all over the place.

Anonymous said...

That 1900 mile trip which he cited would take 27 hours of driving at 70mph and 35 hours at 55mph. That would mean an extra day on the road which would easily cost $150 for hotel, meals and the like. That makes that $70 saving look silly.

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