The 11% figure is from Mythbusters too, there's an actual company, Fastskinz, which builds much more subtle looking coverings which failed to make the grade in one test: Fastskinz Test Drive: Can a Golf Ball Covering Improve MPGs? - Popular Mechanics
Along the way, we periodically checked the fuel-economy readout on the dash display of both vehicles. At 139.9 miles, the Fastskinz Flex was returning 27.2 mpg while the unwrapped Flex showed 28.4 mpg. At 271.1 miles, the Fastskinz Flex was delivering 23.7 mpg and the unwrapped Flex showed 24.1 mpg. We drove until the gaslight glowed in both cars, which turned out to be 430 miles. Back at the gas pump, we filled each Flex in the same manner we did that morning. The Fastskinz Flex returned 24.52 mpg and the unwrapped Flex returned 24.55 mpg. The dash display read 24.8 mpg in the Fastskinz Flex and 25.6 mpg in the unwrapped Flex.
Essentially, in our test, we found no real fuel-economy improvement from the Fastskinz MPG-Plus wrap. And if you trust Ford's MPG displays, the Fastskinz Flex actually delivered slightly worse fuel economy on our loop. So two identical vehicles, on an identical route at identical speeds, with the same drivers, on the same day, returned nearly the same fuel economy. Where did MPG-Plus go wrong?