# Are there any methods to figure out how aerodynamic a race car is without a Cd number?

For the top speed of a car, I believe it's usually limited by drag and horsepower. However, I've also learned that gearing can affect the top speed and acceleration of a car. Are there any methods to figuring out how much more aerodynamic one car is to another only using speed and power? I know this sounds unclear so I'll provide an example: The 2009 Corvette C6.R has 590 hp and can reach 200 mph. The 2008 Aston Martin DBR9 has 600 hp and can reach 180 mph. If you give the C6.R 600 hp, its new theoretical top speed would be 201.1 mph. Does mean that it is more aerodynamic, or is there more to it?

• There is more to it. Commented Sep 16, 2021 at 12:07
• I can guarantee that gear ratios and final drive are causing the 20mph difference in these two cars, not aerodynamic drag. Commented Sep 16, 2021 at 17:22
• I believe it's usually limited by drag, weight and horsepower. Commented Sep 16, 2021 at 21:15
• @chili555 On a flat surface, weight has little to do with top speed. Commented Sep 20, 2021 at 16:33

## 2 Answers

There is a test called coasting which is done by some car magazines. In this test, they reach the car to a certain speed (usually 100 km/h), put it in neutral and let off the throttle immediately and measure how much time it takes for the car to come to a complete stop. If the car travels more distance (and takes more time) before coming to a stop, it means that it has less air resistance. This test not only shows aerodynamics of a car, but also shows other negative forces acting on a car such as wheel bearings friction, tire friction, etc. However, this test is only accurate if you compare two cars at the same time and in similar weather conditions and in the same road. Otherwise, results could be different and unreliable.

• Which is why REAL engineers use wind tunnels. Commented Sep 16, 2021 at 12:58

Ever studied aerodynamics using coefficient of drag formulas in theoretical calculations? Aerodynamics was adapted to racing cars, land speed attempts, NASCAR, dirt track, F1 and drag strip racing. Raising horsepower isn't the only answer to higher speed as drag limits top speed no matter how much hp is added. Drag can be seen as two dimensional, frontal drag as seen standing a few feet in front at waist level, calculating total frontal surface area that creates drag against forward motion thru air. Try walking forward with a 4x8 piece of plywood with the flat face into a breeze then do the same with the narrow thin side into the breeze. The flat portion has a larger frontal surface area and requires more force to walk into a breeze. Walking with the thin side into the wind is much easier and takes less force to walk. Trial and error using mathematics mixed with wind tunnel tests eventually results in a lower Cd. F1 is between 0.7 and 1.1 using a great deal of tuned down force to enter turns at high speed. Cars with 0.25 and 0.3 Cd can't enter turns at high speed unless downforce devices are used. Suvs between 0.35 and 0.45 Cd.