Why do 250 mph (400 km/h) supercars have 0.40+ Cds?
 

Bugatti Veyron Cd 0.41 normal 0.36 lowered and preventing rear spoiler from deploying
Bugatti Veyron - Wikipedia, the free encyclopedia

Hennessey Venom GT 0.43 rear wing raised, 0.36 rear wing down
First Look at the New Hennessey Venom GT - Photos and Just-Released Details - Road & Track

Koenigsegg One:1 0.45-0.50
Koenigsegg One:1

They don't make THAT much downforce. At least not compared to all out racecars.
Are high Cd figures from cooling drag?

downforce to keep them on the ground.

regards
Mech

This is why:

https://www.youtube.com/watch?v=SFN_Gp1eHN0

-soD

Another example

https://www.youtube.com/watch?v=UJIK_e2a5Vs#t=35

It can't help. The Veyron has seven radiators and intercoolers.

I don't think it's downforce in all cases, the Koenigsegg CCX doesn't have much downforce, and neither does the McLaren F1 I think, though they don't hit 250mph.

The particular cars you listed have a lot of cooling drag thanks to the giant turbo motors.

Also keep in mind, fat tires. A 911 turbo S has Cd 0.31 claimed, even though it has only a tiny spoiler and an otherwise well-streamlined body, but it has huge 305 tires.

Considering any type of active downforce... those aren't bad numbers.

A winged racing car has typically two to three times as much drag as that... but it'll be producing a whole lot more downforce.

I would disagree with cooling drag, and I would say it's primarily downforce. These cars aren't designed for normal use.

Downforce is not just about keeping a car on the ground... Nor is it all about reducing the force of wind resistance. It's about being able to transfer 500-800 horsepower to the ground.

Nascar has a CD of .4, Indy car's are even worse, and DTM race cars are worse yet. Super cars are often designed in a similar fashion, because their needs are similar.

"To be faster you need power, but there is a limit to how much power you can put on the ground. To increase this limit, force to ground must be applied on the wheels. Increasing weight can do this, but weight makes handling worse and require more power. So we need some virtual weight, we call it downforce and get it from airflow around the car."

Speed and handling require downforce and not a super low CD. Allowing for more power to the ground, overcomes a "poor" CD. At 200mph, i think a 1 ft square creates drag that takes 70 horsepower to overcome. This is why they have 500-800 horsepower. Brute force combined with downforce allow them to be "super cars".

"In current motor racing competitions, including Formula 1, DTM, Indy cars and Touring Car, aerodynamic downforce plays the most important role in the performance of the cars."

Quoted From:
Downforce

Hope this helps ya.

~C

In addition, there's the matter of styling. Their bodies are shaped to fit people's misconceived ideas of what a fast car ought to look like, rather than what really is aerodynamically effective.

why
 

At these velocities,directional stability and safety are paramount to the manufacturers.They want their customers to live to buy another car.
They may do detail optimization to trim drag as long as it does not impact stability/safety.
Plan-views of these cars reveal drag-unfriendly profiles.