Airlines are always obsessing over fuel costs. It's a crucial aspect of their business, after all, and accounts for one-third of their operating expenses. So you might think that all the major airlines do roughly the same things to minimize their fuel use.

But that doesn't seem to be true — there's actually a surprising amount of variation in how airlines burn fuel. That's one takeaway from an interesting recent report (pdf) by the International Council on Clean Transport, which ranked the 15 biggest U.S. airlines by fuel efficiency and found very large disparities:

This chart essentially looks at how much fuel each airline uses for a given level of "transport service." In other words, the researchers tried to adjust for things like the fact that different airlines travel different routes and carry different number of passengers in order to get a more apples-to-apples comparison. And what they found was surprising.

The least efficient airline, Allegiant Air — a low-cost carrier that targets smaller airports — used 26 percent more fuel than the most efficient, Alaska Airlines, to achieve a similar level of transport. This surprised the researchers, who write: "This gap is larger than what might be expected in a mature aviation market during a period of high fuel prices."

So why is there such a huge disparity? The report examines a few possibilities:

-- Differences in technology: About one-third of the variation likely comes from the fact that different airlines use different technology — they don't all deploy the most advanced, efficient aircraft. Allegiant, for instance, has a fleet of McDonnell Douglas aircraft that dates back to the 1970s. Alaska Airlines, by contrast, uses newer Boeing planes that have technologies like "winglets" to reduce fuel burn.

-- Differences in operations: But technology can't explain all the disparity in fuel efficiency. The report found that some airlines, like Southwest, manage to operate older aircraft quite efficiently. Other airlines, like Virgin, have newer aircraft but are relatively inefficient.

There seems to be a big variation in how airlines operate. Some of this is is just an unavoidable function of the fact that different airlines fly different types of routes. But there's also a big variation in behaviors that should, in theory, be easier to control. Some airplanes might carry too much fuel on flights, weighing them down. Different airlines may have different practices when they taxi their planes on the runway. And so on.

-- High oil prices don't necessarily drive fuel savings. There's another curious angle here: The report points out that the most efficient airlines aren't necessarily the most profitable. Allegiant was the least-efficient airline in 2010 but also the most profitable. That's because it tends to serve airports that other airlines neglect, giving it more leverage to raise prices on routes.

Indeed, the report argued that other factors often play a much bigger role in determining an airlines' profitability — from labor contracts to maintenance costs to fuel-hedging strategies. For that reason, high oil prices don't always push airlines to become more fuel-efficient.

"Fuel prices alone," the report concludes, "may not be a sufficient driver of in-service efficiency across all airlines."

So why does any of this matter? The authors of the ICCT report argue that these disparities are more than just an idle curiosity. Investors may care, for one. But airlines are also starting to come under pressure from policymakers for their effect on global warming — air travel accounts for an estimated 5 percent of humanity's climate impact, and it's a fast-growing source.

Recently, the European Union floated a proposal to require airlines to pay for their carbon emissions. Opponents of the carbon fee argued that the airlines are already operating as efficiently as possible and there's not much more room for improvement. This new ICCT report is essentially saying, "Actually, no one really has any idea if airlines are operating as efficiently as possible."

The report's authors argue that there's "surprisingly little information" about airline fuel efficiency or why there's so much unexplained variation between different airlines. The ICCT report is an attempt to fill in some of those gaps, although because much of the airline data is private, they couldn't fully explain the variation — this was more a "first step" in answering the question.

(Thanks to Climate Central's Bobby Magill for a pointer to the report.)