I am with you, my regular commute variation could be as much as 5mpg due to traffic, weather conditions and unintentional drafting. If you hand measure at pump then add pump variation.
So to get accurate data you either have to do a controlled experiment or rely on statistics.
While mpg gain is speed dependent iI suspect the gains are not linear. VG work by increasing flow speed, so they will lower stall speed of spoiler. If the speed was 40mph and it was reduced to 30, then the biggest gain to be expected in 30 to 40 range. Without knowing car aerodynamics it is hard to guess what to look for and what to test.
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View my fuel log 2015 Mirage DE 1.2 manual: 46.4 mpg (US) ... 19.7 km/L ... 5.1 L/100 km ... 55.7 mpg (Imp)