re drag coefficientsvx s in a previous article gwm spl spl loral com gary w mahan says could someone explain how to make sense of drag coefficients i e cd mentioned in magazines i understand that lower numbers signify better aerodynamics but what does this mean in the real world is there a way to calculate new top speeds assuming the car is not rev limited at top speed or mileage benefits if a identical car had the cd reduced from to it s pretty complex and cd isn t the whole story either cd for cars is usually calculated based on the frontal area of the car so a large car with a good cd could get the same drag force as a smaller car with a poorer cd to calculate drag use this formula d rho v cd s where d is the drag force lbs rho is the local air density slugs ft v is the velocity ft s and s is the frontal area ft note that the pieces called rho v are sometimes called qbar or dynamic pressure a fancy aero term for air pressure or force note that power is p f v where p is power lbf ft s f is the force drag in this case lbf and v is velocity ft s note that if you put the whole equation into one by substituting d for force you get a velocity cubed term that s why huge increases in power result in little increases in speed ditto for decreases in cd so if you have a mph car and reduce cd from to your new top speed is sound of trumpet fanfare mph sorry to dissappoint al bowers dod alfa ducati hobie kottke blad iaido nasa well goodness sakes don t you know that girls can t play guitar mary chapin carpenter
