The average American car is overpowered because we like those zippy starts and we just gotta have the passing capability. This mean extra or reserve HP. American engines are vastly overrated as far as HP goes. Divide the factory rated HP by two and you get what the engine will really put out.
The average car about 3000 lb will need 20 to 30 HP to make it roll. That will go up to about 60 or wherever max RPM on motor is. Then you have to add on more HP to overcome wind resistance and even more if you are climbing hills.
If you design for 60 mph and for a 20% grade you should have a fairly good performing vehicle.
