Wind Mileage Factor

How much warmer was it outside on the way home than on the way there and how much colder was the car starting out? Might be a contributor.

 

wind is just air speed. if you are in a 15 mph headwind driving 55 mph your car will use about the same amount of gas as you would driving 70mph. tail winds work the exact same way in reverse.

 

I experienced exactly the same thing on Sunday in Minnesota. I drove 125 miles into a 25 to 40mph wind. Fuel economy was somewhere between 30 and 33mpg. The car blew around, but so was everyone elses. I did not sense anything unusual about the handling in such strong winds.

 

Here in Manitoba the wind seems to blow constantly, and it always seems to be a side wind or a head wind. For that reason nobody believes the highway MPG rating, as they know they'll never achieve it.

 

Air drag is only part of the equation. There is also tire rolling resistance, drive train resistance, the drag of the internal gearing, and some kind of emf back drag in the electric motor. None of these are linear. The effect isn't exponential but their non-linear increase helps explain why mileage suffers greatly above 50-55 mph,

 

wind resistance is about 100% greater at 70 mph than 50.

from the site How stuff work.com

Drive slower - The aerodynamic drag on the car increases dramatically the faster you drive. For example, the drag force at 70 mph (113 kph) is about double that at 50 mph (81 kph). So, keeping your speed down can increase your mileage significantly.

http://auto.howstuffworks.com/hybrid-car19.htm

 

Aerodynamic drag increases with the square of velocity, and the power required to overcome that drag increases with the cube of velocity (drag is a force, work is force * distance, power is work per unit time or F * v)

So to go twice as fast you need 8x the power. Going from 50mph to 70mph takes 2.75x the power to overcome aerodynamic drag.

The formula to calculate aerodynamic drag is something like:

F = 1/2 * air density * coefficient of drag * frontal area * velocity^2

Rolling resistance, including drivetrain losses, increases linearly with velocity.

Try riding a bike into a 20mph headwind. That should give you some idea of how quickly aerodynamic drag dominates rolling resistance.

 

drivetrain losses do not increase in a linear fashion according to the testing done on the prius by the oak ridge national lab. I don't think rolling resistance is linear either.

 

well at least you could understand my explanation... although most of it was plagerized...: