Dallas billionaire T. Boone Pickens and FedEx Corp. chief executive Fred Smith are now duking it out—over, of all things, the virtues of natural gas as a transportation fuel.

Since announcing the “Pickens Plan” in July, the oilman-cum-wind power booster has spent over $60 million, along with countless hours zig-zagging the country in his corporate jet, to promote his plan for using wind power and natural-gas vehicles to break the country’s foreign-oil habit. The Oklahoma-born oil magnate insists the U.S. could cut its oil imports by one-third in 10 years by mandating that all new long-haul trucks dump diesel in favor of liquefied natural gas.

But Mr. Pickens has his opponents, including FedEx CEO Fred Smith, who favors electrification of the transporation fleet. Mr. Smith argues that hybrids are the way to go, and is putting his money where his mouth is. With 80,000 motorized vehicles, FedEx now boasts the largest fleet of commercial hybrid trucks in North America.

Without naming Mr. Pickens, the company’s director of sustainability, Mitch Jackson, upped the ante on Sunday with a blog item blasting natural gas as transport fuel of the future. After citing a list of reasons against using natural gas instead of diesel, Mr. Jackson concludes that “substituting one fossil fuel for another may mean we’re shifting our energy supply, but it doesn’t necessarily mean we’re going anywhere.”

Mr. Pickens then let rip with a rebuttal that accuses Mr. Jackson of making a “flawed argument” by misunderstanding the country’s natural-gas reserves and overstating the value of diesel hybrids.

“Not only does Jackson need to do more homework on the domestic availability and clean air benefits of natural gas,” Mr. Pickens writes in his Daily Pickens blog, “he needs to realize that deploying vehicles that use slightly less foreign oil – vehicles that have little testing or are not available in the marketplace – will not solve America’s energy crisis.”