The Postal Service has been testing out a cleaner vehicle fleet for many years. It is today made up of over 30,000 vehicles like hybrids (pictured) and ones that run on CNG or biodiesel, among many other gasoline alternatives. The USPS is also trying other ways to save fuel, like better delivery routes. As Sustainable Business reports, though, the USPS' flexfuel vehicles not only saw a decreased fuel efficiency of 29 percent, but also ended up forcing the USPS to use 1.5m gallons more gasoline than before. Why? Because the USPS couldn't buy and use E85 everywhere it wanted to and so the flexfuel engines - which were larger than the ones they replaced - were thirsty and burned more fuel. The USPS has declared that it will now only use E85-capable vehicles in places where the biofuel is "competitively priced and conveniently available," a USPS study says (see page 17 of that PDF for easy-to-understand maps of what went wrong here).
[Source: USPS, Sustainable Business via EVWorld]