[M_a_t_t]

To answer your question though you might try the toned down top speed test. The throttle stop test. Just limit your throttle to (just guessing here) 50% and see what that top speed is. Then mod your car and re-test using the same limitations.

I am curious if this would be accurate over long periods of time. I wouldn't want to have to remove all of my mods and go back to a stock car to compare a new mod, so maybe you could collect some speed data and use that as a base forever, or just work backward from your previous change.

Then you can do the math to calculate the change* in cd based on change in top speed.

I don't find the math very difficult to do, just need the data mostly.

*which means you would need a starting point to be able to say your cars cd went from .3 to .25 rather than 17%