SEATTLE (AP) _ A sleek, red car with doors reminiscent of gull wings drove from Mexico to Canada on $15 worth of gas, winning a fuel efficiency rally with an average of 103.7 miles per gallon.

The Avion, driven by its designers, Craig Henderson of Bellingham and Bill Green of Berkeley, Calif., beat the prototype Viking VI from Western Washington University, which finished Wednesday with an average 89 mpg on the 1,759-mile trip.

''We were a bit surprised ourselves at the car's performance and mileage,'' Henderson said. The high-performance Chrysler gasoline engine normally in the hand-built, $30,000 car was replaced with a Volkswagen four-cylinder diesel engine for the Three Flags Econorallye event, he said.

The car beat last year's record of 88.2 mpg.

The best mileage the Avion logged was 114 mpg from Eugene to Portland, Ore., Henderson said. The rally began Aug. 5 and finished Wednesday, with the cars set to appear Friday on display at Expo 86 in Vancouver, British Columbia.

The rally was organized by the Fuel-Efficient Vehicle Association of Seattle and sponsored by the oil company Unocal. The group was founded by alumni of Western Washington University's Vehicle Research Institute. The drivers of the Avion and Viking VI were current or former students at the institute.

''The purpose of the rally is to show that a car built with today's technology can get better than 100 mpg,'' said association spokesman Bill McRae. ''We want to provide a test environment for people with ideas to improve fuel efficiency by using the best industry technology.

Seven cars took part in the rally. The second-place Viking VI was driven by Mark Small, Dave Clark and Mark Prince of Western Washington. Bill Fraser and Kelly Habeck of Mankato State University drove the third-place Honda CRX, which logged an average mpg of 73.51.

A three-cylinder Chevrolet Sprint took fourth at 65.4 mpg, and was driven by Harold Bisel and Dennis Boos, both of Fresno, Calif. A Suzuki Samurai JX, driven by Dan Wing of Millbrae, Calif., and Walter Taucher of Edmonds, was fifth with 40.1 mpg.

An Oldsmobile Delta 88, driven by automotive journalist Leon Mandel and car expert Kevin McElwain, posted 32.14 mpg to take sixth place. The Cyclone, a radically altered 1972 Opel Manta had an average 23.01 mpg to finish seventh. It was driven by Kim Hall of Bellevue and Kevin Whitman of Seattle.