Ford's F-Series pickup, the most popular vehicle in America, presents large-scale opportunities when operated on compressed natural gas (CNG). Last year, natural-gas vehicles saved 400 million gallons of gasoline on U.S. roads, according to one trade group, and emitted about 14% less greenhouse gas on light trucks.

Costs to convert an F-150 half-ton truck are high, though, ranging between $6,000 to $9,500, but fuel costs are lower, with CNG averaging $1.50 less per gallon equivalent than gasoline, according to the Energy Dept. Fleet managers running high-mileage vehicles say they can see a payback in as little as two or three years.

Starting late last year, Ford has been the only manufacturer to offer a light-duty truck prepped at the factory to run on natural gas. For a $315 option, Ford will harden the F-150's standard 3.7-liter V-6 engine with rugged valves, seats and other components while preserving the truck's original warranty. Dealers will support the trucks and work with authorized up- fitters that install additional fueling equipment and match Ford's warranty. Some upfitters can ship the truck directly to a fleet owner's yard, too.

Converting the truck with gaseous fuel lines, electronic controls and a bed-mounted tank takes some time. If a dealer has a prepped truck in stock, the turnaround can take as little as one week, says Kelly Muldoon, director of business development for Venchurs Vehicle Systems, which supplied the conversion for an F-150 truck that Ford recently loaned me for a week. Lead times are longer if a prepped truck is not readily available. "Order to delivery, you're looking at probably 10 to 12 weeks," Muldoon says.

In most cases, the 2014 Ford F-150 looked and operated like a normal pickup. The Venchurs conversion, which used hardware from Altech-Eco, included a 23.5-gallon-equivalent tank in the bed hidden under a stainless-steel toolbox cover. At the pump, I was able to pipe CNG through a fuel port on the left-hand side of the box. The truck also included a 36-gallon gasoline tank for a combined range of about 750 miles.

A CNG fuel gauge on the top of the dash was sloppily installed—resembling a high-schooler's half-baked work on a hot rod—but Muldoon assured me that Venchurs plans to integrate it into the truck's console. Doing so could add a slight cost but would look a lot cleaner. Below the headlight controls was a rocker switch that allowed me to change on the fly between CNG and gasoline. If one of the tanks were to run dry, the truck would switch automatically to the other fuel.

The transition between fuels was mostly seamless. The truck sometimes shuddered for a moment when it switched to CNG, and the engine's operating noise was slightly louder than on gasoline. Otherwise, the changeover happened quickly and did not slow me down.

Fuel economy was surprisingly good, even though CNG has less energy density than gasoline. The four-wheel-drive truck, finished out with an extended cab, came with a rating of 16 miles per gallon in city and 21 mpg on highway while on gasoline. Driving on CNG, I observed 16.6 mpg in mostly city traffic. During my 2012 test drive of a Ram heavy-duty bifuel pickup, I got less than 12 mpg.

Calculating fuel economy is bit a of a moving target, as the volume of the gaseous fuel can change with ambient temperatures. I discovered this after one hot afternoon filling up the Ford pickup. The next morning, the truck had cooled off, and the fuel gauge suddenly dropped by a quarter tank. I went back to the filling station and squeezed in a few more gallons. The second trip came with extra smiles per gallon, as the fuel cost only $2.24 per gallon-equivalent. Most large cities have CNG pumps available, but more are concentrated in regions with large amounts of oil-and-gas activity, such as Texas and Oklahoma.

The cargo bed loses the most in the deal, as the CNG tank enclosure reduced its effective length by 27 in., about a third of the 6-ft, 6-in. bed. And while the tank box also impairs visibility out of the rear window, it has some useful space underneath its lockable lid.

With this versatility comes a higher acquisition cost. My test truck, which came in medium XLT trim, stickered for $40,515, which included the gas-prep kit and dealer delivery charges. The retail price of the Venchurs upfit cost $8,750, which brought the grand total to $49,265. Incentives are available in some states for those looking to switch to natural gas. And for buyers looking for even higher performance in this size truck, Ford is rumored to have a prep kit in the works for its larger, 5.0-liter V-8 engine.