Source: Westport Innovations.

Free gas!

For two whole years!

With gasoline prices in the U.S. still averaging more than $3.24 a gallon, and soaring as high as $4 in some places, the promise of free gas for two whole years makes for a heck of a sales pitch -- and at Westport Innovations (NASDAQ:WPRT), they're pitching it hard.

This week, Westport announced a new project designed to goose sales of vehicles equipped with its "Westport WiNG" compressed natural gas fuel system. If you buy a certain number of trucks and/or vans equipped for natural gas fuel-use, Westport will pay to keep them full of gas ... for as much as the next two years.

As Westport explains, it's giving away the equivalent of 750,000 gallons of gasoline over the next two years, in the form of compressed natural gas, or CNG. According to the company, truck buyers who take it up on its offer could reap fuel savings of anywhere from $1,195 to $4,780 per truck they buy.

It's good to have friends
Westport isn't going this alone. It's enlisted powerful partners in the effort. Ford (NYSE:F) makes the trucks  -- modified F-150s and F-250s, and running all the way up through the monstrous F-650 model (with E-series vans to boot). Clean Energy Fuels (NASDAQ:CLNE) runs the natural gas fueling stations that deliver the fuel to run the trucks. And of course, Westport makes the WiNGs.

The F-250 with Westport WiNG. Looks like a Ford, runs like a gas furnace. Source: Westport.

The catch
By now, you've figured out the catch(es) to this deal. First, we're talking free natural gas here, rather than actual gasoline. Second, you need to make a pretty big upfront investment to get hold of the free CNG.

Buy at least five WiNG-equipped trucks by Dec. 31, and Westport will provide the equivalent of 500 gallons of free fuel for them -- up through Dec. 31, 2015, unless supplies run out sooner. Buy 10 to 24 trucks, and you can claim 1,000 gasoline-gallons' worth of CNG. Purchasing 25 to 49 trucks will net you 1,500 gallons, and if you buy a whole fleet of 50 trucks or more -- collect 2,500 gallons, free of charge.

That's quite a catch
Yes, 50 trucks will clearly be beyond the budget, or needs, of most individual truck shoppers. But for commercial buyers, Westport's deal may make sense. Clean Energy operates only about 150 CNG fueling stations across the country right now, after all. But if you happen to own a business near one of them, and were already thinking about switching to nat-gas, then the prospect of getting paid nearly $4,800 in fuel savings per truck to convert your truck fleet may sound appealing.

What's more, even after the free fuel offer runs out two years hence, Westport says that thanks to low natural gas prices, fueling a truck with CNG, as opposed to liquid dinosaur-juice, could save an owner more than $2 per gallon-equivalent.

The upshot: The more you drive, the more you save with Westport. And now, the more WiNG-equipped Ford trucks you buy, the more you save as well.

Fool contributor Rich Smith has no position in any stocks mentioned. The Motley Fool recommends Clean Energy Fuels, Ford, and Westport Innovations and owns shares of Ford and Westport Innovations. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.