An automobile is a wheeled vehicle with a self-contained motor that runs mostly on roads. Automobile speed is an outcome of four interacting factors: power, power train, weight and aerodynamics.
The motive force for a car is the engine. Every engine produces an amount of work-energy measured in mechanical horsepower. One horsepower is 550 foot-pounds per second. More power in generation from the engine means more power available to turn the wheels faster on the car.
A car with a 100 horsepower engine still has to have the energy from the engine transferred to the mechanical axle to turn the wheels. The mechanisms that effect this transfer are called the power train. Reduction of friction and resistance throughout the power train will make more energy available to turn the wheels, and thereby increase speed.
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Two equally powerful engines, with two equally efficient power trains, and unequal weights will have different top speeds because of the difference in weight. Lighter weights translate into more speed.
Air causes greater resistance at increasing speed. The faster you go, the more air you are moving in a given amount of time. Air resistance to automobiles increases progressively with speed. Aerodynamic design that reduces wind resistance increases car speed.