It won't be exactly 5% on the full scale vehicle, but it also wont be too far off. The only time you can assume that the drag coefficient of the car also goes down by 5% as long as the transition point is the same and that the Reynolds number is being matched.
If they are trying to designing a car, then they care about how to reduce drag. They'll probably use a smoke wand and/or tufts for flow visualization.
If, on the other hand, they are trying to improve an existing car, then they should already know where the full scale cars boundary layer transitions and use trip strips at that spot. Then the numbers should match up fairly well (provided the Re isn't in a range so low that it becomes sensitive to Re).
I don't do car aerodynamics, but since a car is a very unaerodynamic shape I would think the flow over it is not very laminar most of the time anyways. Cars are designed based on styling (for the most part) since rolling resistance causes more drag than the air does for speeds below 60mph.
I still strongly recommend that you write out an example problem doing dimensional analysis to see for yourself.