[Phase]

Originally Posted by aerohead

I'd give up on trying to quantify delta-CdA for anything small. That's what wind tunnels are for.
If, say, you have 'smaller', 'higher fineness ratio', and 'further-distanced' mirrors from the car, you might just presume that there's an advantage.
You really can't afford to 'know.'
Old-school road testing requires a thermal stabilization period of 30-minutes, driving 50-mph, before you ever begin the actual testing.
1) Calm ( or not to exceed SAE specs )
2) Dry
3) Straight
4) Level
5) 10 runs, back-to-back, with each configuration
6) Continuous time-stamped weather monitoring
7) 1/10th-second recording resolution.
8) Coasting through 70-mph, down to 20-mph
9) 5th-wheel data transmitter ( mechanical or optical )
10) Data-logger/ strip recorder
11) Done to SAE Handbook specifications.
12) You may prefer root canals to the mathematics required for data reduction.
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I bailed, and just paid CAR and DRIVER to do the tests at the Chrysler Proving Grounds. Between that, and Bonneville, and lots of tank mileage, I eventually got some numbers that told the tale.

For some of my more mild mod testing, I’ll be driving out past the Columbia gorge over and hour away on a stagnant air day. Engine should be warmed up and I should be able to do coast down testing starting at 90 mph. It’s middle of nowhere and long and flat. May also try some throttle stop testing on top of the coast down testing just to cross examine the results