Oldsmobile has substantially redesigned its flagship Ninety-Eight for 1991, coming up with a luxury sedan that melds traditional American buyer tastes with European-inspired road manners.

That in itself is not unprecedented. Nearly all American luxury sedans have been put on crash diets in recent years, becoming leaner, meaner and more responsive in hopes of rivaling some European offerings.What is surprising is how different the new Ninety-Eight is from the Buick Park Avenue, its soul mate at General Motors.

Both cars have been joined at the hip for years in basic design, sharing the same platform, running gear and many styling cues. That caused GM major embarrassment during the mid-1980s, when one could no longer tell a Buick from an Oldsmobile.

Although these nameplates still share the same drivetrains and a fair amount of underpinnings, their exteriors are more diverse than ever for 1991, putting an end to the "look-alike" era. About the only shared exterior part is the windshield.

While the Park Avenue has a sleek, handsome profile, the Ninety-Eight gets the more distinctive, formal look.

Its slab-sided flanks and turret-top roof may remind some of a 1950s Nash, but its overall styling is more modern than retro because of the swept-back windshield and ribbed plastic cladding on its lower body.

The new Ninety-Eight is almost 10 inches longer than the previous version, with most of that dedicated to added rear-leg room and a trunk that is 23 percent larger.

It also is 280 pounds heavier than the 1990 version, but fuel economy is unchanged due to improved aerodynamics and engine performance.

Two models are offered for 1991. The Ninety-Eight Regency Elite starts at $23,695, replacing the 1990 Regency model that started at $19, 995.

Most conveniences expected of any luxury car are standard, plus a driver-side air bag, adjustable-height shoulder belts, Teves anti-lock brakes, AM-FM stereo cassette, cruise control, solar glass and anti-theft system.

The Ninety-Eight Touring Sedan begins at $28,595, up from $26,795. That makes the Touring Sedan Oldsmobile's most expensive car ever and the first one to break the $30,000-mark if equipped with a few options.

Amenities like a leather interior with walnut trim, keyless entry system and steering wheel-mounted controls for the stereo and ventilation are standard, as are larger tires on 16-inch rims and tighter steering for crisper road response.

Few extras are offered: an electric sun roof ($1,230), compact disc player ($396) and heated seats ($235). The model evaluated had no options, costing $29,175 after just the $580 destination charge was added.

That price may certainly raise a few eyebrows. But although aimed at domestic luxury competitors like the Lincoln Continental and Cadillac Seville, the new Ninety-Eight Touring Sedan comes closer than ever to some pricey luxury sedans from across the Great Pond in terms of both ride and handling.

It has more of a carved-from-a-single-piece feel than before, thanks to a stiffer body shell and underbracing. Much road harshness and wind noise has been tuned out from its 110.8-inch wheelbase, while road "feel" has been tuned back in, partly through a stiffer steering system.Drivers will find the Touring Sedan's hooded instrument panel another blend of American and European tastes.

There is even a tachometer among its wide array of needle pointer gauges. But to the right are no fewer than 49 buttons to operate the ventilation system, stereo and trip computer.

All Ninety-Eights have GM's 3.8 liter "3800" V6 engine that delivers 170 horsepower at 4,800 rpm. It is coupled to GM's all-new, electronically controlled 4-speed 4T60-E automatic transaxle.

The engine has ample acceleration for a sedan weighing nearly 3,700 pounds, and its transaxle does indeed have nearly imperceptible shifts. Observed fuel economy, however, was 20.9 mpg after mostly highway driving, despitEPA ratings of 18 city/27 mpg highway.

The Ninety-Eight evaluated had a high level of fit and finish, with no discernible flaws. Each one is backed by a three year/50,000 mile bumper-to-bumper warranty and Oldsmobile's 24-hour roadside assistance plan.