[basjoos]

Some of World Solar Challenge runs in Australia have averaged speeds over 60 mph (won by the Nuna 3 and the Tokai Challenger) and some of the cars are capable of sprinting to speeds over 90 mph.

If you are going to install a slant back nose to lower the stagnation point and already have a smooth underbelly, then an air dam likely won't do much to reduce your drag given its attendant increase in frontal area since you have already cleaned up the areas under the car that the air dam deflects the wind away from. But you would just have to experiment and see what your results are. On my car I have a rounded nose which extends forward and downwards the curving slope of the hood and has a profile like the front of an airfoil.