Just a couple days back - there was a recent story: DIY ‘Boat Tail’ Gives Little Car Big Fuel Economy - By Chuck Squatriglia, Autopia, about Darin Cosgrove, an aquaintances Geo Metro Aero Mod package!
His Car - a Pontiac firefly - Like the same one I am using for my Electricfly, but newer, has had simple but practacal testing done - and recorded by Video camera and Tuft Testing to measure flow and turbulence, as well as multiple driving runs to test with mods on and off, to verify the differences, has shown at least a 15% improvement on an already very fuel efficient vehicle.
He Says - “I’ve driven with the tail on for about 300 km in total, mostly at highway speeds up to 100 km/h,” Cosgrove told us. “I didn’t notice any change in handling. The only effect on driving is I have to watch out for rubberneckers in other cars. Seriously.”
The Article mentions "The tail joins the rear wheel skirts, bellypan and other mods he’s made to boost aerodynamic efficiency. The result isn’t much to look at, but it boosted Cosgrove’s fuel efficiency by 15.1 percent at 90 km/hr (56 mph)."
These are all elements to show - that - a vehicle which usually gets below 5 Litre per 100 km, and is now getting below 4 litres per 100 km range - means that's now about one less trip needed to go to the gas station each month or so, so there is a time saving portion to the result as well as a money saving part!
Follow the story to see the Video he posted at the end.
Just think - what would you save if you cleaned up the bottom and rear of your vehicle? Also - if it's done right - you gain a large rear end cruple zone to protect your self and your fellow occupants!