here's an article from 2004:
USPS Delivers - With AFVs
The United States Postal Service (USPS) runs the largest federal fleet, with approximately 208,000 vehicles. Of those vehicles, USPS operates approximately 7,400 alternative fuel vehicles (AFVs). The majority of these AFVs are converted "long life vehicles" (LLVs), designed with a 1,000-lb capacity and a 24-year life span. By converting a large portion of these vehicles to compressed natural gas (CNG), the USPS has created the nation's largest fleet of CNG delivery vehicles. The USPS also has other medium- and heavy-duty trucks that operate on alternative fuels.
Early in 1998, the USPS purchased 601 light-duty CNG or E85 (85% ethanol, 15% gasoline) AFVs for purposes other than to deliver mail. These original equipment manufacturer (OEM) vehicles include Ford Motor Company models of the ethanol flex-fuel Taurus, the CNG bi-fuel Contour, the CNG bi-fuel 1/2-ton and 3/4-ton pickups, the CNG bi-fuel 1-ton van, and the CNG dedicated 3/4-ton van.
Then USPS made another bold move in September of this year, purchasing 10,000 E85 Ford Explorers that will deliver mail. The Ford vehicles will be custom built with right-hand drive and fitted with the USPS delivery body.
In 1999, USPS plans to purchase a number of electric vehicles (EVs), and will seek cost-sharing from local governments, utilities, and various other sources (the majority of these EVs will be located in Clean Cities communities). They also have long-term plans to convert all 128 trucks operating from the Dallas, Texas, bulk-mail center to run on liquefied natural gas (LNG).
snip
USPS has been working with the ethanol industry to expand the ethanol infrastructure in the D.C. area where E85 vehicles will be located. �"With the addition of Ford E85 vehicles, more ethanol infrastructure will be established by the ethanol industry, which will be used by the USPS as well as other government agencies, such as GSA," said Marguerite Downey, AFV Program Manager, Environmental Management Policy, USPS. "The smartest deployment program (for ethanol vehicles) is to work with other federal fleets."
much more here
http://www.eere.energy.gov/cleancities/ccn/archive/2_5feature.html