Frequent fliers, listen up. Tuesday is the “cheapest” day to use your airline miles. And you’re most likely to find the lowest-mileage domestic coach-class award tickets on Southwest and Delta.

At least that’s the finding of a new report out by MileCards.com, a website that lets users compare travel credit cards based on where they want to fly. The group says it analyzed “over 100,000 award travel date and route combinations among the frequent-flier programs of the five biggest U.S. airlines.” The five biggest U.S. carriers are American, Delta, United, Southwest and Alaska Airlines.

The MileCards.com analysis looked exclusively at domestic awards for economy class travel. Among the notable findings:

– On average, frequent-fliers will expend the fewest miles or points for award travel on Tuesdays. MileCards.com says “across all airlines studied, prices on Tuesday are 26% lower on average than those on the most expensive day,” which is Sunday.

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– Southwest’s Rapid Rewards and Delta’s SkyMiles were cheapest for awards in the U.S., according to MileCards.com. Its analysis found Southwest charged an average of 13,629 miles for a round-trip award. “The catch,” MileCards.com says, is that “Southwest doesn’t fly to Hawaii, or some popular award destinations in the mainland U.S. like Vail and Anchorage.”

Delta is something of a surprise at No. 2. The carrier’s SkyMiles program, derided in many frequent-flier circles for the perceived poor value of Delta miles, showed an average round-trip domestic award price of 27,871 miles across all routes studied, according to the analysis. Delta’s average dropped to 20,0001 miles round trip when looking only at routes where Southwest also operates flights – a more limited subset.

Perhaps sensing the disconnect between Delta’s strong performance and its reputation among frequent-flier enthusiasts, MileCards.com offered an explanation in its analysis.

IN PICTURES: Behind the scenes at Delta (story continues below)

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“Over the last few years, Delta has stopped publishing a menu of award prices, and widened the range of prices it charges for awards. Now, domestic economy coach tickets can be found on many routes for just 10,000 miles round trip, and others at…