I'd guess that between 1923 and 1938 Ford (the most-popular make used) didn't do too many aero tests on their sub-sub-sonic roadster bodies -- which usually went down the road with a big top up.
And, as for use in Modified Roadsters, their aero probably depended on the shape of the builder's body fashioned from the firewall forward. Hardly any two are alike.
If someone gave you a clue on their body, it probably wouldn't apply to someone else's same-year body -- and not be reliable to use for comparisons to other years only.
You've also gotta look at Streamliners -- where there are hardly any restrictions -- and notice that almost all of them are also unique. (I'm not much help here, am I?)