General Motors made a mistake.

Hard to believe, isn`t it?

GM brass snickered and guffawed when they learned Ford Motor Co. wasgoing to bring out new midsize sedans, called the Ford Taurus and MercurySable, in December of 1985 as 1986 models.

GM had prototypes in clay of its new midsize models: two-door, sporty W-body coupes. Sedans were for old fogeys; coupes were what was happening,dude, reasoned trendsetting GM executives, to whom ``casual`` means leavingwing-tips untied.

Ford got the last laugh. Consumers tossed aside their canes and lined upto buy the stylish sedans with rounded sheet metal, rather than GM`s boxyintermediates such as the Chevrolet Celebrity.

Still, GM was committed to its W-body coupes, and in the fall of 1987,about two years later than planned, the midsize cars appeared as 1988 models. But a funny thing had happened on the way to the showroom. Before introducing the cars, GM had to tweak the sheet metal a bit, because the prototypes lookedlike they came out of the Taurus/Sable design studio. Amazing coincidence.

Lookalike cars are one thing when an Oldsmobile looks like a Buick lookslike a Cadillac. But lookalikes don`t sit well with consumers when an Oldslooks like a Ford looks like a Mercury-especially when the Olds wasn`t thefirst.

Despite the problems, the GM cars won many kudos . . . from the generalmanagers of the Buick, Oldsmobile and Pontiac divisions. But consumers didn`t get caught up in the cheerleading at GM and bought the Ford sedans.

Last fall GM righted a wrong and brought out W-body sedans in the Olds,Pontiac and Chevy lineups. The last to debut, but perhaps the best, is the1991 Buick Regal.

When we first drove a crudely thrown-together prototype last summeroutside Detroit, we could only suspect that Regal would prove to be aboveaverage when all the bolts were tightened and the wiring fastened.

We`ve now had the chance to drive a production model in Chicagoland andconclude that if the Regal sedan had appeared at the same time as Taurus, GMmight have gored the Ford bull.

What sets the Regal sedan apart from the other GM W-body cars is smoother styling that emphasizes subtle curves of the sheet metal, rather than aharshly sloping roofline such as on Grand Prix and Cutlass Supreme.

What makes it an even more special machine is the addition of the Buick3.8-liter, 170-horsepower, V-6 engine, which provides a kick when moving offthe line, away from the light, down the merger ramp or into the passing lane. The 3.8 packs a punch you don`t get from the 135-h.p., 3.1-liter, V-6 that`sstandard in the Regal and offered throughout the rest of the W-body line. The 3.8 is powerful yet quiet, a very good combination.

Taurus, by comparison, offers a 3-liter, 140-h.p., V-6 as standard and a140-h.p., 3.8-liter, V-6 as an option. Though the horsepower is the same, you experience the burst at lower r.p.m. with the 3.8, V-6. Still, Regal enjoysthe horsepower edge.

The 3.8 is a $555 option at Ford, a $395 option at Buick. Another plusfor Regal.

The 3.8 teamed with 4-speed automatic is rated at 19 miles per galloncity/28 m.p.g. highway in Regal and Taurus, a draw.

Regal also offers optional antilock brakes, which ensure quick, accuratestopping regardless of road surface. When introduced, Taurus didn`t offerantilock brakes, but they have been added as an option for 1990. Antilockbrakes cost $925 in Regal, $985 in Taurus. Another Regal edge.

To cater to sound-system devotees, the `91 Regal sedan offers not only an AM-FM stereo, but also places a set of controls in the dash and the steeringwheel hub for quick use without taking one`s hands from the wheel.

The system is a step up from the similar one at Pontiac, in that Regal`soffers an on/off switch along with those to change stations and volume. Yet,for the ability to play with your radio while making a left turn, yousacrifice an air bag in the steering column. Taurus lacks steering-columnradio controls but gives you an air bag. Score one for Ford.

We test-drove the top-of-the-line Regal Limited, which comes with a hostof standard equipment, including 4-speed automatic; power brakes and steering;``dynaride`` suspension, which combines just enough stiffness to feel thepavement with the right amount of softness to keep from being tattooed by tar marks; dual, remote sideview mirrors; air conditioning; tilt steering; tinted glass; stainless steel exhaust; steel-belted, all-season, radial tires; andbody side moldings.

Our test car had a ``prestige`` package that for $2,936 added powerwindows/seats/door locks/antenna, remote trunk release, electronicinstrumentation that makes gauges and digital displays difficult-to-read artworks, cruise control and rear-window defogger.

Base price was $16,120. With the options mentioned, leather bucket seats($450), remote keyless entry in which a push of the key fob locks orunlocks the doors ($125) and others, the Limited sedan stickered at $21,051,to which you add $475 in freight.

The Regal sedan offers the performance, room, comfort and quiet youexpect in a mid-size luxury car. Ride is a bit smoother, handling slightlylivelier, off-the-line performance quicker than Taurus.

But there are some changes that would make it more pleasurable.

The rear seat, as we`ve said with all GM W-body cars, comes up short onthigh support because the bottom stops right about where the briefs do, bethey cotton or silk. Long-distance travel wasn`t designed into that seat.

Then there are the shoulder/lap belts that slide out from the driver andpassenger doors, a safety system from the GM-Gillette school of design thatgives the neck and cheek a close shave.

You also have to wonder how the engineers and stylists could have used asplit-level dash, which provides a valley for ill-placed coins and pens aswell as dust.

On the plus side, there are such pleasant features as a cupholder thatlifts out of the center console, a safe spring-held hood, space-savingashtrays that slip in and out of the rear walls, a roomy trunk, an easy-to-usehood-release lever below the driver`s seat and a windshield so expansive youfeel as if you`re captain of a Greyhound bus.

And give Buick credit for putting a hood ornament on the Regal. Withoutone you`d look out the windshield and see nothing between you and the car/garage wall/pedestrian ahead, because the hood is sharply sloped. Theornament gives you a perspective of distance.