IMPERIAL
The air resistance against a vehicle is a force in lbs equal to .0025 times the miles per hour squared times the frontal area.
AR = 0.0025 x MPH2 x FA
AR = Air resistance in lbs
MPH = Speed in miles per hour
FA = Frontal area of vehicle in square feet
Example:
What is the air resistance of a vehicle travelling 40 miles per hour and having a frontal area of 80 square feet
AR = 0.0025 x (40)2 x 80 = 320 lbs
METRIC
The air resistance against a vehicle is a force in newtons equal to .0462 times the kilometres per hour squared times the frontal area.
AR = 0.0462 x KPH2 x FA
AR = Air resistance in newtons
KPH = Speed in kilometres per hour
FA = Frontal area of vehicle in square metres
Example:
What is the air resistance of a vehicle travelling 65 kilometres per hour and having a frontal area of 7.5m2?
AR = 0.0462 x (65)2 x 7.5 = 1464 N