I was hopeful for another tank of over 39 mpg, but alas...
Still, I am riding a streak of pretty good gas mileages. The average of my last five tanks is 38.30 mpg, which is the fifth-highest average for the year (32 tanks, so it's in the top 15.6%).
This tank has raised my average fuel economy for the year up to 37.86 miles per gallon, just 0.01 miles per gallon less than my all-time high average fuel economy that was set in 2007.
It's also looking like this year will see me break the all-time annual gasoline expense. So far I've spent $1,226.36 on gas (average $3.388/gallon). Last year's record cost totalled $1,332.35, and I still have 2.5 months left in the year. Quick estimating in my head of 4 tanks/month @ $35/tank = ~$350, so the predicted total annual gasoline cost of ~$1,575 will totally eclipse my current record.
So I got that going for me!
So, after two tanks of driving with the duct tape over the lower part of my front bumper, my mileage doesn't appear to have improved by any notable amount. But two tanks does not a complete experiment make, so I'll continue. If anything, the deflected air will allow my engine to warm up faster as we enter the cooler months.