How much power (and MPG) does fake ram intake kill?

[DreamV4]

Side covers (ram intake) definitely look nice but they probably decrease MPH by 10%, especially at high speed.

 

[Fire-medic]

That is a very-pessimistic number you put out there.

Swim trunks and tennis shoes will net you an additional 6.9 MPH

 

[VMAXXIMUM]

All I can think of is rolling and sliding around, getting all rashed up by flipping salt. Gives fresh meaning to “feel the burn”.

 

[MaxMidnight]

Side covers (ram intake) definitely look nice but they probably decrease MPH by 10%, especially at high speed.

On what basis do you make that assumption and how have you arrived at the 10% figure?