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Jim, I agree, the Insight's fuel economy is particularly twitchy. You can control the assist/regen with the clutch switch mod, and that will save lots of gas, too. But you should test in neutral.
A-B-A test results do not have to be reported in mpg. It would be more relevant and accurate to report the change in your CdA after, before, and after an aeromod, or in your low-speed rolling distance A, B, A tire mods or weight reduction.
If all you have is a long, steady grade, you might be able to tease a CdA number out of it, but the math is much easier when you eliminate the gravity term by testing either on a perfectly flat road (which does not exist), or on a mostly flat road in both directions.
If you are testing powertrain mods, such as a hot air intake or an IMA inhibit switch, obviously you can't test in neutral. I would log your commute mpg, with and without the mod. Large sample size is the key to getting meaningful information out of noisy data. However, if the expected impact is only a few mpg, although the mod may be worth doing, the sample size required to determine the impact of the mod may be prohibitively large.
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