Contrary to what Chevy, Dodge and Nissan would have you believe, Z is not the last letter in total performance. M is.

If I were allowed only one car for the rest of my life, I'd choose the BMW M5 because it feels as if it will never wear out. That feeling of solidity and sturdiness emanates from the high quality of the materials used in the car's fit and finish, its powerful and precise drive train, its aircraft-quality brakes, and to a slightly lesser extent, its impeccable road manners.

The BMW M5 offers the best of all possible worlds: plenty of interior room, as you would expect from a big sedan; loads of performance, as you would expect from a sports car;

and the comfort and convenience you would expect from a luxury car.

Though many manufacturers have tried to muscle in on BMW's territory - most notably Nissan with the Infiniti Q45 - it's the German automaker that still is defining the standards for which sports sedans are judged.

ENGINE, TRANSMISSION, PERFORMANCE

Under the hood lurks what must be the most powerful, straight six automobile engine in production today: a 216-cubic-inch, 310 horsepower, DOHC, 24-valve dynamo that propels the M5 from 0 to 60 in just 6.1 seconds and to a controlled top speed of 155 mph. Impressive? You bet. We are not talking about a sports car here, but a 3,804-pound sedan.

Much of the power generated by the M5 probably could be traced to a set of wild camshafts. At idle, the M5 is a bit rougher than you would expect of a BMW. Whereas you can't usually discern a BMW engine at BMW, G-??

M5 is a bit rougher than you would expect of a BMW. Whereas you can't usually discern a BMW engine at idle, the M5 rumbles and grumbles and lets you know it's ready for action. Once under way, though, it smooths out quickly.

BMW equips the M5 with a five-speed manual transmission only. You let out the clutch and touch the accelerator - and are somewhat underwhelmed. But when the engine winds up a bit, the power (and speed) multiplies exponentially. The M5 howls when floored, such as when accelerating onto an interstate. You reach the speed limit quickly and with ease - usually only one shift from first to second is required.

The transmission, shifter and clutch are excellent, but there was one trait in the test car that was bothersome. In first gear with the air conditioning on, the car was very jerky. Letting off on the accelerator at, say, 10 mph, was enough to knock driver and passenger forward in their seats.

The Environmental Protection Agency rates the M5 at 11 miles per gallon in city driving, 20 mpg on the highway and 14 mpg combined. I drove the M5 very hard in the city and got exactly 11 miles per gallon. However, in highway driving I did slightly better with a respectable 21 mpg. The M5 takes only unleaded premium.

STEERING, HANDLING, BRAKING

It's not just the M5's engine that has been given a thorough going-over b y BMW Motorsport, the company's competition and sports division. The wheels, tires, brakes, suspension and steering also have been upgraded from that of the regular 5-series sedan.

You would think that the M5 sedan, weighing nearly 2 tons, could get unwieldy in some situations. It doesn't. I'm convinced there's nothing you can do to undermine the M5's handling. For one thing, there's nearly 3 feet of rubber on the ground. The low-profile tires are 17 inches in diameter and 8 inches wide, ensuring plenty of grip.

The power-assisted steering is superb. The M5 is agile, easy to maneuver and spectacular in high-speed turns. The four-wheel, anti-lock disc brakes could stop a Boeing 747.

FIT, FINISH, CONTROLS

One of the things I liked most about the M5 was the way the controls, switches and buttons felt. It is these - more than anything else - that give you the impression the M5 is built to last. If BMW pays as much attention to the design of an air con itioning or light switch, you can only imagine how well the engine is put together.

And the air conditioning, by the way, features separate controls that let the driver and passenger adjust the temperature. The AM/FM/cassette sound system in the bright red test car featured an optional ($780) trunk-mounted CD player.

The black leather seats in the test car were extremely firm but also very comfortable. The rear seats, divided by a console with a storage compartment, also were comfortable, but not especially inviting for tall or large passengers.

The M5's large windows afford excellent visibility and never give the driver a claustrophobic feeling. In 1990, BMW exported only 1,000 M5s to North America. No decision has been made this year, but it is not likely that more than 3,000 or so M5s will be built. They are designed, I think, for those totally enamored by the BMW marque.