Originally Posted by howard

Interesting post. Where do you get the 30666 and 11089 from? Can you expand on the calculation please.


The formula for calculating the energy needed to overcome aerodynamic drag is

0.5 x Air Density x CdA x Speed**3

Air Density can be ignored for comparative purposes, CdA is the drag coefficient multiplied by the frontal area of the vehicle in m**2

All values in SI units, so 50 mph is 22.3m/s which cubes to 11089.567. 70mph is 31.3m/s hence 30666

The result of the formula is expressed in Joules/second, ie Watts. Divide by 746 to get HP of course.

Note that although aerodynamic drag increases with the square of the speed (which is what Peter quite correctly said, and I rather sloppily corrected by not using clear enough terminology), the power expenditure necessary to overcome drag increases with the cube of speed. For a detailed explanation of why this is, see:

https://physics.info/drag/

Tim H.
1986 4/4 VVTi Sport, 2002 LR Defender, 2022 Mini Cooper SE