delta 10% Cd = delta 5% HWY mpg
 

For what it's worth,
I ran across an online paper by Douglass et al., from the University of Delaware, USA, in which they were still using the 10=5 rule-of-thumb as of 2006.
Hucho has something close to it in 1986. It's a bit more liberal for Diesel-powered vehicles.
William H. Bettes, used it in his researches in the Graduate Aerodynamic Laboratory California Institute of Technology ( GALCIT ) wind tunnel, reporting in 1982.
General Motors' Aerodynamics Laboratory was using it in the early 1990s.
'Highway' is not to be construed with 'EPA HWY' protocols.

There are plenty of references around that show otherwise. I've cited them before and I can't be bothered doing so again.

As soon as you see a rule of thumb like this in car modification - any type of car modification - you can be almost certain it's wrong.

Rules of thumb don't work well in car modification - especially numerically precise rules of thumb.

wrong
 

The data is presented for informational purposes only. What the reader derives from it is solely up to the reader.

You should put that in your signature. It would save everyone a lot of trouble!

should
 

Great wisdom should be practiced also by the purveyor of it.:)

should
 

computer crashed before post could be submitted. Lost my train of thought.