Detroit — A decade ago, full-size cars had the ''aerodynamics of bricks,'' asserts Gary Romberg, manager of the aerodynamics department of Chrysler Corporation. Chrysler at the time had three engineers working part time on aerodynamics. The job, Mr. Romberg says, was to reduce air resistance on Plymouth and Dodge stock racing cars, not the run-of-the-mill highway cars that motorists bought from a car dealer's showroom. It was a time when the full-size cars built by the company had a ''drag coefficient'' of around 0.60.
It just didn't matter when gasoline cost 40 cents a gallon or less. However, the 1973-74 Arab oil embargo and resulting energy crisis rewrote the script entirely.
Now Chrysler has six engineers working full time on future cars, none of them for the race track, with an eye on a coefficient of drag (CD) of 0.20 to 0.29 - a measure of the ease with which air passes over, under, and around a motor vehicle.
Aerodynamics has always been vital in aircraft and space-vehicle design, but only recently has the auto industry learned that even at 35 to 40 miles an hour, aerodynamics has a profound influence on the miles per gallon a car will deliver.
One carmaker goes so far as to insist that aerodynamics has a bearing on fuel economy as low as 15 m.p.g., while another says that aerodynamics influences m.p.g. at all vehicle speeds.
Two things impede the movement of a vehicle over the road, asserts Art Lewry of the Ford Motor Company - rolling resistance and wind resistance.
To improve miles-per-gallon performance, automakers are attacking both problems. In modern cars, wind resistance and rolling resistance are just about equal at 40 miles an hour. But as cars become more aerodynamic, the speed at which the coefficient of drag equals the rolling resistance will be higher.
Improved tires and higher tire pressures also reduce rolling resistance.
Consider the following:
* To improve fuel economy by 1 mile a gallon by using high technology alone costs about $1 billion. But achieving the same result with better streamlining costs a small fraction of that amount.
* Revisions to a vehicle so as to improve its aerodynamic qualities cost an automaker virtually nothing in the way of materials because it only involves the shape of the materials and not their choice or quantity.
* For every 10 percent reduction in highway fuel economy, there is a 3 percent improvement in metropolitan highway economy and 2 percent in city mileage, according to engineers at Ford, Chrysler, and General Motors.
* At the present time, most cars produced in the world are in the CD category of 0.40 to 0.50. By the mid-1980s, however, most cars built here and abroad for sale in the United States will be in the range of 0.30 to 0.40. By the end of the decade, some vehicles will have a CD of between 0.20 and 0.30.
* By the mid-'80s many cars will have the air intake for engine cooling purposes placed below the front bumper, a position found to add the least to the CD. The below-bumper air intake enables the two-seater Mercury LN7, for instance , to post one of the industry's lowest CD figures on a production-model vehicle - an impressive 0.34.
* Within a few years all vehicle windshields will be flush-mounted (a few already are) for minimum wind resistance in a very critical area. Rear glass also will be flush-mounted, although the CD improvement is far less than in a flush-mounted windshield.
* Side glass as well will be flush-mounted before long. One production car, the Northern Ireland-built DeLorean, already has flush-mounted side glass; while another prototype model, the Isuzu Piazza, built in Japan but not exported, uses flush glass all around.
Many motorists still object to side windows that are not movable, according to A. Lloyd Nedley, aerodynamics coordinator for Chevrolet passenger cars, who agrees that the Isuzu Piazza is an excellent example of flush-mounted side glass. However, he adds, flush side windows are much more difficult to achieve than flush-mounted windshields.
* Windshield mounting angles will become more rakish until 65 degrees is reached. There is no need to achieve an even higher degree, because there is no further aerodynamic gain by increasing the angle.
Nearly all cars, domestic and import, will have flush-mounted windshields by 1985, using the type now found in the Cadillac Eldorado; all of the GM X-body cars, such as the Chevrolet Citation; and the six-month-old J-cars.