IMPERIAL
The horsepower required to overcome air resistance is the speed in miles per hour, cubed, times the frontal area in square feet divided by 150,000
Where:
MPH = Speed in miles per hour
FA = Frontal area in square feet
HP = Horsepower
150,000 = A conversion constant
Example:
What is the horsepower required to overcome air resistance of a vehicle travelling 40 miles per hour and having a frontal area of 80 square feet?
METRIC
The power in watts required to overcome air resistance is the speed in kilometres per hour, cubed, times the frontal area in square metres divided by 77.86
Where:
MPH = Speed in kilometres per hour
FA = Frontal area in square metres
P = Power in Watts
77.86 = A conversion constant
Example:
What is the power in watts required to overcome air resistance of a vehicle travelling 65 kilometres per hour and having a frontal area of 7.5 m2?