Hi Darcane!
Well, in spain we do the soapbox racing in public streets ( closed during the event) and the track is pretty long, watch this video:
---https://youtu.be/nTqPZygdRRU---
My soapbox is unfinished yet, the max speed that i reached is about 60 km/h ( about 40 mph), but the top level ones reach 95 kmh (near 62 mph).
i'm not sure too about how to calculate the cd, it exists some empirical method?
some advice?
some drawings of the body shape: