EVcast #46: EEstor Explained - To the Extent We Can

Based on a

1000 kg car, frontal area 2.3 m^2, Cd=0.34 (modern car), Crr=0.01

fd=0.095*(Vmph)^2 N

frr=98.1 N

N=Neuton which is about 5 lbs of force

speed-rolling resistance-air resistance

10mph-98N-9.5N

20mph-98N-38N

30mph-98N-86N

40mph-98N-152N

50mph-98N-237N

60mph-98N-342N

70mph-98N-466N

80mph-98N-608N

For a story, when I was like 12 driving across country in a Chevy van in the 1970s.  I kept track of gas mileage.   I remeber we used a tank in Yellowstone where the max speed was like 35 mph and remember getting great gas mileage.

also Cd=0.25 for the insight

Hi Bill, thanks for the all the Math Mr. Einstein :)  If I am understanding this correctly, the slower you go, the better the gas milage?  At some point, the use of onboard electronics (including AC) and pure engine idle must factor in.  So the question still remains, what is the ideal speed to drive for best gas milage?

Bo,
   Bill's explanation is great for the load to push the car down the road, but perhaps misses something about the efficiency of the engine operating in these different conditions.

    I looked for some graphs on the web based on experimental data. Generally 45 MPH is the most efficient speed although it ranges from 35 MPH to 55 MPH in some cases.

    What is clear is that for everyone doing 75 MPH on the interstate, they are not getting optimal fuel efficiency and the national 55 MPH speed limit was probably effective at reducing fuel consumption (if enforced).

    If you look at Bill's data, somewhere at 35 MPH, the wind resistance becomes higher than the rolling resistance.  This is a key cross-over point.  Above this speed, wind resistance starts to kill the car's efficiency.  It also shows why drag coefficient (Cd) is so important.  This is controlled by the shape of the car.

  Here are some graphs.

http://casitaclub.com/forums/uploads/post-4856-1185328649.jpg

  This suggests optimal speed of 35-45 MPH based on an average of 15 cars from the 1980's.

http://www.randomuseless.info/318ti/mphmpg.png

   This is a 1996 BMW with optimal speed of 40 MPH (look at the upper line in the chart).

http://www.cleanmpg.com/photos/data/522/MPG-Vs-MPH.JPG

    This is 1999 Accord showing an optimal at 45MPH

http://i38.photobucket.com/albums/e101/FogerRox/mpg_vs_speed.jpg

    Another study showing 40-45 MPH as the optimal (but one data point showing 55 MPH as optimal)

 http://www.fsec.ucf.edu/en/consumer/transportation/conservation/images/Speed_vs_Mileage-500px.gif

Once again 40 MPH is the optimal