The best time to book flights for your next getaway may be six weeks before you want to travel, upending the widely held belief that buying earlier leads to better deals, according to a new report by Airlines Reporting Corp.

Looking at any 120-day advance purchase time period last year, the study found that travelers paid 5.8 percent below the year's average fare of $358.30, when they purchased tickets six weeks in advance of the desired departure date. The flights analyzed for this study had origins and destinations within the U.S. and did not include sales of tickets purchased directly from airlines.

Airlines Reporting Corp., based in Arlington, Va., provides data, analysis and financial settlement services for the airline industry, and is a clearinghouse of  193 airlines and travel sellers, including websites and local and corporate travel agents. The company said it looked at data from 144 million transactions worth $82 billion in ticket sales throughout 2011.

While the six-week advance purchase date seems to yield the best results, the company does not recommend travelers only purchase tickets at that time, since there are no guarantees of the cheapest price. But Chuck Thackston, managing director of data and analytics with Airlines Reporting Corp., said several years of previous data also show that purchasing six-weeks in advance leads to the cheapest flights.

But other airfare watchers show slightly different data from with Airlines Reporting Corp's findings.

The golden rule of airfare shopping for Rick Seaney, CEO of Farecompare.com, is to buy domestic airline tickets on Tuesdays at 3 p.m. Eastern time as the airlines scramble to match cheap flights filed Monday night until the maximum number of cheap seats hit the reservation systems.

Seaney said Farecompare.com's data shows airlines start actively managing their cheapest domestic seats about 3 1/2 months before departure and that window closes 14 days before the departure date. That's when legacy airlines start treating customers like business travelers who can afford three times the early bird price.

And seasonal airfare may vary widely from average prices.  Travelocity's Pricing Barometer identified July 28 to Aug. 3, as the best week to find cheap flights for last year's Labor Day weekend, based on its four-year historical data.  And Expedia's data for domestic standalone flights in 2011 indicates that it is best to buy airfare 22 days prior to the intended departure date.