XL Hybrids, the powertrain-electrification company founded in 2009 by graduates of the Massachusetts Institute of Technology, will start selling a plug-in hybrid add-on designed for fleets of pickup trucks. The company says its XLP "upfit" is specifically for half-ton pickups, whose models include the best-selling Ford F-150 . XL estimates that such systems can boost fuel economy by as much as 50 percent compared to conventional powertrains.The company has reached fleet-purchase agreements with public-utilities companies such as the San Diego Gas & Electric Company, Liberty Utilities, Hawaiian Electric Company, and Montgomery County Maryland, and will start delivery of the product by the end of next year. XL says the upfit can be installed in "just hours" and offers a three-year, 75,000-mile warranty on they system, which leaves a truck's engine, transmission, and fuel system fully intact. XL also will offer a cloud-based analytics system that fleet managers can use to measure fuel economy and cost savings.The Seattle Fire Department is among the municipality divisions that have recently adopted XL's technology . XL said in September that Seattle's Ford E-450 ambulances that were retrofitted with the hybrid add-on had achieved a 23-percent boost in fuel economy in nine months of driving, while emissions were down 19 percent.In 2013, XL launched its XL3 hybrid powertrain designed for General Motors and Ford vans. The company estimated at the time that the hybrid powertrain, which adds about 350 pounds to the vehicle, would boost fuel economy by about 21 percent and would save fleet owners as much as $15,000 in refueling costs during the life of a single vehicle.: A previous version of this headline confused the XL3 and XLP units, and incorrectly noted the amount of possible fuel savings.