aerodynamic advantages of a clean car would be very high.
The difference is in the fact that a newly washed car, the surface is slick and glossy and smooth, air slides right off of it, as the car gets dirtier the gloss fades and the surface of the car becomes rough, air does not like to travel over rough materials.
I dont know the order of magnitude it would change, but it would change, and enough to make a difference in a top speed test easily. The slightest change would make a difference over 60 miles per hour.