Tether car is interesting idea

I had one of those once upon a time, Cox 049 powered

but, not sure how statistical data can be sampled from it...
Wind tunnel I understand, I can play with smoke - just ordered a Casio exilim for some high speed video, and I can sample pressure zones via probe, and get some force numbers to try to work back to a drag level...
The question becomes can one work out close enough in a 1:24 scale without hitting near Mach speeds in a huge tunnel, where improvements are needed and get a rough idea on how those modifications translate to full scale?