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The easiest good resolution test I have found is from a terminal velocity test. But the biggest problem with that test is it is hard to find a place to do it. For my best hill it is a mile or two at a 5% grade with not a lot of traffic most of the time and the wind always blows the same direction at about the same speed.
Terminal velocity was good enough to tell a difference in the rear wheel covers and things that made a marginally measurable change to mileage. For the stuff that was lost in the noise I had to do a more precise test.
For the absolute best resolution for my testing I would top the hill at 55mph and push the clutch in at a sign post and start coasting. I would hit terminal velocity and keep coasting down the hill at the bottom of the hill there was a sign that I would note my speed. For example if I was doing 60mph crossing the sign. Add something minor like the rear spoiler and I would do 61-62 at the sign at the bottom of the hill.
I used that most days coming home from work to let me know if a tire was a few psi low or if something wasn't quite right with the car. If I was really careful with the test I could see a difference with the back windows just being popped open or not.
The speed at the bottom coasting test works good because it gives the car a long time for the small aero changes to work on the speed of the car. So terminal velocity might only be 0.3mph faster and impossible to measure at 90mph but the slightly better aero made it coast a bit longer on the flat bottom part of the road and that is much easier to measure. It was also very repeatable and variations in runs were pretty much nothing even through several days/weeks for me.
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