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ABA...
A Test without mod.
B Test with mod.
A Test without mod again.
This is to make sure that any gain in economy with the mod isn't due to differences in run conditions (temperature, traffic, wind, etcetera).
Of course, it's not perfect. Some people do ABAB, AABBAA, or even ABABAB.
It's all down to how much time/money/patience you have.
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Tuft test is attaching little dangly things ("tufts") that move in the wind (string, paper streamers, whatever), and photographing/videoing them as you drive the car along. (Errh... best if you have someone follow and photograph/video you.) This lets you see how the air moves along the body of the car... and identify areas that might be improved with mods.
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