Once done, we found that even with an oil change, clean air filter and proper tire pressure, we were averaging roughly 9.4 miles to the gallon.
…
And guess what? With the device on, we were now averaging 23.2 miles to the gallon. That's 61% better than the gas mileage we were previously getting.
Um, from 9.4 miles per gallon to 23.2 miles per gallon is a
61% increase?!?

What kind of advanced math is that? My calculations show that it is more like a 147% increase.
A 61% increase to 9.4 miles per gallon is
15.1 miles per gallon, not 23.2 miles per gallon.