Holy smokers!! My last '68 (289, 2BBL, C-4) gave me a steady 16MPG in the city. I drove it to work daily, not expressway roads either. MPG dropped to 14when I ran the A/C, which in Miami was quite often. Highway MPG was 18, running the A/C - I had the evaporator sensor set so low it would blow ice particles out just prior to freezing up!!
I haven't checked my C/S mileage yet, but now I'll make it a point to do so.
As far as the rear spoiler effect, I don't believe there is any aerodynamic benefit. It just looks great.
If I recall correctly, I once had it up to 65MPH. Wow, what a rush!! Good nite all.
Neil