Thirty years ago, General Motors' head stylist, Harley Earl, unveiled the "Y-Job," a prosaically named but revolutionary harbinger--power-operated convertible top, extended front fenders, concealed headlights--of what GM had up its sleeve. Thus was born the dream car. Since then, the Motor City's blue-sky production lines have turned out idea autos in ever-increasing numbers; almost every Detroit make was represented in this year's auto shows either by some far-out vehicle boasting its name plate or--at the very least--by a highly modified, "futured-up" version of a current model. The "one-off" dream car can be an expensive proposition, but the car companies amortize its cost in a number of ways. Except for pretty girls, there is no surer method of drawing the crowds over to an auto-show display than to have a dream car as the exhibit's focal point. In many instances, the car is used by the company as a barometer with which to gauge reaction to contemplated changes. As Dodge General Manager Robert McCurry put it at the introduction of the Charger III: "This experimental vehicle is our way of showing the public some of the design and engineering concepts that we have developed. From the public we learn what it would like, or not like, to see in tomorrow's automobile." (Recent futuristic display items heralding assembly-line realities were the Mako Shark II, which bore many of the design characteristics of the current Corvette, and American Motors' "Ramble"-seated AMX, which wound up, sans rumble seat--but with almost all of its design intact--as the production model.) There is also the dream car's high-gloss glamor, some of which the makers hope will rub off on its more pedestrian bread-and-butter brethren. This is often accomplished by the simple and inexpensive expedient of transferring the dream car's name to one particular model or to a whole line of production autos. GM's Biscayne, Le Mans, LeSabre and Firebird; Ford's Mustang, Futura, Cougar and Monterey; and Chrysler's Dart, Newport and Adventurer are examples of Detroit's retain-the-name game. Conversely, American Motors' Tarpon--by some mysterious piscatorial machination of the consumer surveyor's craft--passed on most of its looks but wound up in the dealers' showrooms as the Marlin. Occasionally, automotive suppliers such as U.S. Steel, Borg-Warner, Bridgeport Brass and Dow Chemical get in on the dream-car act, coming up with advanced-styling vehicles to help pitch the use of their products in auto manufacture. Although Detroit continues to investigate alternative means of powering its vehicles--rotary-piston, gas-turbine, electric and even steam engines are being weighed as means of propulsion for the car of the future--it's obvious that what could wind up under the hood doesn't fascinate the show going public nearly as much as a way-out auto body. Which is why the daringly designed dream car is here to stay.