Horsepower & Aero Drag Calculator
By David Nguyen

Assumptions: Vehicle is a car driven on concrete with tire pressures 30 psi. Ambient temperature around 25 degrees Celsius (77 F), no wind.

Drag figures are usually calculated from wind tunnel models without side mirrors, gaps in the windows and body panels, and smooth wheel covers. These all contribute to Cd and can increase frontal area. Most of the drag on a car is aero dynamic drag rather than rolling resistance (from tires and weight).

Top speeds are either gear limited or drag limited. Use the Gear Ratio Speed Calculator to determine which is the case for your particular scenario.
 

Reposted October 10, 2006