Get more airline reward miles for paying less — but only for a limited time

As of this week, all four of the nation’s biggest airlines — American, Delta, Southwest and United — dole out miles to their loyalty-reward members based on how much the fliers spend on airline tickets instead of how many miles they travel.

By converting to a program based on spending, the carriers give more rewards to travelers who buy the expensive front-of-plane seats while cutting back on points for travelers who buy discount or sale tickets. American Airlines was the last of the giants to make the switch, on Aug. 1. United Airlines and Delta Air Lines changed their reward programs last year, and Southwest Airlines converted in 2011.

But now one ultra-low-cost carrier is taking the opportunity to promote itself and fire a shot at the big guys.

Spirit Airlines, the super-cheap Florida-based carrier with the long menu of passenger fees, launched a promotion this week that hands out more loyalty-reward miles to travelers who spend the least on airline tickets. A traveler who spends less than $75 on a flight gets 1,000 miles while a flier who spends between $126 and $200 gets 150 miles.

The promotion runs only until the end of August.

“We want to fly in the face of changing loyalty programs and reward customers for saving money, not spending it,” said Bobby Schroeter, vice president of consumer marketing at Spirit.

That may sound like a good deal except that Spirit’s reward miles expire after three months of inactivity, while the other airlines give travelers one or two years of inactivity before they erase the points.

“The biggest challenge for Spirit is that once you fly with Spirit, you might not want to do that again,” said Zach Honig, editor and chief of ThePointsGuy.com, a loyalty-reward program comparison site.

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