LEAD: ''The Continental, introduced in 1939, was one of the most graceful automotive designs ever executed, a fitting aesthetic sign-off for the age of the American classic car.''

''The Continental, introduced in 1939, was one of the most graceful automotive designs ever executed, a fitting aesthetic sign-off for the age of the American classic car.''

Stephen W. Sears in ''The American Heritage History of the Automobile in America.''

This seems to be my week for Lincoln Continentals. First of all, I have been driving a new one. Then, just by chance, I discovered the hulk of a classic 1956 Continental Mark II on the lot of a New Jersey restoration shop and spent some time running eyes and hands over its lovely lines. That same day, I also spotted a beautiful metal model of the 1956 in an antiques shop and dug into my pocket for more than I could afford.

The only part of this that was planned, in fact, was the test drive of the silver Continental, which seemed a good idea on a couple of counts.

This year is the car's 50th birthday. And the test seemed to have further merit because, like some other cars today, the Continental's dash provides information on instant and average fuel economy, not a bad thing given the price of gas and the probability that Washington's budget proposals will add further tax to every gallon.

The first Continental was conceived by Henry Ford's son, Edsel, as a special car to use on his vacation in Palm Beach, Fla. Returning from a trip to Europe, he gave sketches of foreign luxury cars to Ford stylists and asked them to give his car a ''continental look'' that would include a spare tire mounted on the trunk.

The vacation vehicle was delivered in February of 1939, and everyone who saw it thought it was sensational - so much so that the company decided to put it into production. The 1940 model, introduced in October of 1939, was built on a Lincoln Zephyr platform, had a 120-horsepower V-12 engine and cost $2,850.

The car has long been a collector's item, and prices now range upward of $30,000. The nameplate disappeared in 1949, but that second Continental, the Mark II, produced another instant classic when it was introduced in 1956, and that car, made only in 1956 and 1957, was priced at $10,000, lofty indeed when you consider that a standard Lincoln hardtop could be had for $4,600.

For 1991, the front-wheel-drive Continental is a four-door sedan in the middle of the Lincoln line, and it comes in two versions, the Executive series at $30,211 and the Signature series at $32,120.

Standard on both are such things as antilock brakes, computer-controlled air suspension, heated sideview mirrors and cruise control. But the Signature models add molded aluminum wheels, combination door locks and a power trunk, among other things.

Upholstery is in leather, with buckets up front and a bench in the rear. There is good room, but lack of storage is a surprising problem. The glovebox is a medium-size bin, containing a single cupholder in its lid, and there are pleated pockets on the doors, but that's it. No place for tapes, none for compact disks. Nowhere for a driver's coffee cup, and no handy place for toll change.

The driver does get an air bag in the hub of the four-spoke steering wheel, and the dash is built to accommodate a passenger air bag as well. However, Ford has had problems getting the larger right-side air bags (the supplier's plant blew up), and a company spokesman said they won't be available until December.

Behind the leather-wrapped wheel, you look at a digital dash whose speedometer paints itself in inch-high green. Buttons on the right side of the panel control the ''Message Display Center'' and show trip mileage, instant economy, average economy and how far you can go on the fuel in the tank. And that gets us back to what turned out to be an exercise in economical driving.

The Continental's 3.8-liter V6 is rated at 18 miles per gallon, city, and 26, highway, and three days' of mindless commuting provided a baseline average of 21.4 m.p.g. - not bad for a car that weighs 3,663 pounds but an average I thought could be improved upon.

With the message display set on instant economy and the constantly changing digits kept above the baseline, the highway figure rose to an amazing 31.6. And despite New York's impossible traffic and ever-present construction, careful driving produced an overall average of 22.8.

That was more than a mile per gallon better, and the process made the commute a lot more interesting. I did wonder, though, what Edsel Ford would think about today's luxury and the notion of fun with feather-footing.

Photos: The original 'vacation car'; In 1956, an instant classic; The 1991 Continental Signature series.