Here we go again. Once again, Clean Energy (Nasdaq: CLNE) shareholders are watching the value of their investment spike. Once again, they have Big Transport to thank for it.

Since touching bottom at $12.40 on June 27, investors have thrilled to a 7% rise in stock price. Last time something like this happened, it was in response to news that Clean Energy had inked a deal to supply a UPS (NYSE: UPS) depot with fuel for 48 natural gas-powered trucks. This time, the catalyst appears to have been a contract that Westport Innovations (Nasdaq: WPRT) inked with General Motors (NYSE: GM) to collaborate on the development of natural-gas engines for light trucks.

Bull thesis
Now, Westport is not Clean Energy, but there's still logic to this run-up for the latter, on good news from the former. There does seem to be some momentum behind a move to using natural gas to fuel road vehicles. Texas oilman T. Boone Pickens is a big backer of the trend and has invested in nat-gas plays such as Chesapeake Energy (NYSE: CHK) in anticipation of it. ExxonMobil (NYSE: XOM) also lent a vote of confidence when it acquired XTO a few years ago. If GM, too, is moving toward nat-gas power for its trucks, this will naturally boost demand for nat-gas fuel -- and for Clean Energy's business of building, operating, and supplying nat-gas fueling stations. 

Bear thesis
Q.E.D.? "Not necessarily," reply the bears. As fellow Fool Rich Duprey pointed out last month, "GM won't have a fleet of Westport-inspired nat-gas cars hitting the roads anytime soon." A tie-up with GM might boost revenues for Westport in the short term, but any bump in business for Clean Energy will have to wait for GM to perfect the technology, build the trucks, and convince people to start buying them. Only then will there be a need for Clean Energy to supply the fuel to get 'em moving.

Problem is, while Westport's deal increases the visibility of a future for nat-gas vehicles in the long term, Clean Energy's own future looks considerably cloudier in the short term. While (barely) profitable today, Clean Energy shares sell for a pretty high multiple to trailing earnings -- 71 times, to be precise. Worse, things look likely to get worse before they get better. Analysts are predicting a decline in profits next year, boosting the stock's forward P/E up to 83.

Long story short: There still seems more downside to this stock than upside.

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Fool contributor Rich Smith owns no shares of any company named above, but The Motley Fool owns shares of UPS, and Motley Fool newsletter services have recommended buying shares of GM, Chesapeake Energy, and Westport Innovations.

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