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There are a lot of variables other than the vehicle itself to account for that affect mpg: driving habits, speed, temperature, wind, traffic. In order to determine what effect your aerodynamic mods have, you have to control all other variables.
Doing my best to control those, I am seeing about a 3.5% increase with my grill block and belly pan. Assuming a new max of 18.6mpg from your previous best of 17.5mpg, you are seeing about twice as much improvement. I suspect only part of it is from the grill block. Especially since the consensus seems to be that blocking the upper part of the grill is more effective than blocking the lower parts.
It still helps, but don't expect miracles.
Mike
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