Farecast.com, a new travel Web site that claims it can forecast the rise or fall of air ticket prices, plans to help travelers solve the long standing riddle of how to catch fast-moving and often illogical fare deals.

The site, which went live on Tuesday after about a year in beta testing, says it can predict with 75 percent accuracy the direction of air fares over seven days.

Timing is important when booking travel, because discounted fares are generally only available for about 50 hours, said Farecast Chief Executive Hugh Crean, in an interview.

Farecast, which supports itself through advertising and referral fees to airline Web sites, is a new breed of travel site aimed at rivaling established players such as Expedia Inc. and Travelocity.

The site backs up its predictions with a product called FareGard, which costs $9.95 and guarantees the low price for a week. If the price paid is higher than guaranteed price, Farecast pays the difference.