Can you tell me where you got the constant 168 from and what it represents?
Don't know where I got it, probably a car magazine from the 60s. There is a lot of necessary numbers juggling to get from A to Z in that formula--168 rolls it all together. I never bothered to break it down. You can if you want.
Also if the speed is not drag limited does this mean I can fit a massive flat board on my car to increase drag force and given an infinite and straight road it will reach the same top speed as it would without the massive flat board at the front?
By the definition of the term "not drag limited" it will.
However, if it wasn't drag limited before adding the flat board, it may be now.
If a car is not aero drag limited in it's top speed capability, it means that there is a mechanical or other limitation to it. Namely- the car is so unstable that the driver doesn't want to push it any faster, the engine reaches a given speed (redline on the tach) and the drivers lifts off the gas, or the engines rev limiter cuts the power.
Or there is a police blockade in your alley.