Quote:
The power required to overcome the aerodynamic drag is given by:
P_d = \mathbf{F}_d \cdot \mathbf{v} = {1 \over 2} \rho v^3 A C_d.

Converted to layman's terms
There are amount of power (energy) needed to push your car through the air is
For 60mph:
It takes 27% more energy to push the air aside when you're driving 65 instead of 60
It takes 59% more energy to push the air aside when you're driving 70 instead of 60
It takes 95% more energy to push the air aside when you're driving 75 instead of 60
And 65:
It takes 25% more energy to push the air aside when you're driving 70 instead of 65
It takes 54% more energy to push the air aside when you're driving 75 instead of 65
So the short rule is: At freeway speeds increasing your speed by 15 miles an hour roughly doubles the amount of gasoline required to overcome wind resistance. You can save a whole mess of gasoline by slowing down five or ten miles an hour.
Another factoid: Most cars get their best gas mileage at speeds between 47 and 54 miles per hour.
So, here's the table comparing efficiencies at speeds over 55mph:
It takes 30% more energy to push the air aside when you're driving 60 instead of 55
It takes 65% more energy to push the air aside when you're driving 65 instead of 55
It takes 100% more energy to push the air aside when you're driving 70 instead of 55
It takes 154% more energy to push the air aside when you're driving 75 instead of 55
And, even more impressive, for 50 mph:
It takes 33% more energy to push the air aside when you're driving 55 instead of 50
It takes 73% more energy to push the air aside when you're driving 60 instead of 50
It takes 220% more energy to push the air aside when you're driving 65 instead of 50
It takes 275% more energy to push the air aside when you're driving 70 instead of 50
It takes 336% more energy to push the air aside when you're driving 75 instead of 50
See the
[b]BIG jump at 65 mph? We, as a country, could save a whole mess of gasoline and reduce our greenhouse gas emissions by a
[b]HUGE amount by setting our metropolitan area (where the highest number of freeway miles are driven) speed limit to 50 or 55 mph and our rural area speed limits to 60 mph.
Science stuff:
Spelled out mathematically, the amount of energy required to push the air aside is:
1/2 the thickness/pressure of the air
multiplied by your car's windface (the total area of your car or trailer facing into the wind)
multiplied by the amount of aerodynamic drag for your car/trailer (more aerodynamic = smaller number)
multiplied by your car's speed
multiplied by your car's speed again
multiplied by your car's speed a third time
Shortened even more, the amount of energy needed to push your car through the air is
(a bunch of stuff you can't control)
multiplied by your car's speed
multiplied by your car's speed
multiplied by your car's speed
Peter
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