If you can't understand why pickup trucks have become the nation's hottest selling vehicles, all you have to do is motor on down to your local Chevrolet dealer and take one of the best trucks you can buy - the Chevy C1500 Sportside - for a test drive. It used to be that trucks were strictly for work. They were utilitarian affairs with vinyl seats, no carpet, manual windows, and maybe an AM radio. Their rides were stiff. These days trucks such as the Chevy C/K series are quiet, comfortable, stylish and luxurious, but - as always - sturdy and hard-working. At this moment, two trucks are locked in a dead heat for the title of best-selling vehicle in the United States for the 1993 model year. In sales, Ford's F150 has the Chevy edged out by a few thousand vehicles. Ford beat Chevy in sales for the 1992 model year, but this time Chevrolet has a good chance of closing the gap. That means you are likely to find that Chevy dealers will be willing to be somewhat flexible with the prices in order to move these trucks. PERFORMANCE Our test truck came with Chevy's optional 210-horsepower, 5.7-liter, fuel-injected V-8 and a computerized four-speed automatic -two options that added $1,735 to the base price. Smooth, powerful and extremely quiet, the 5.7-liter V-8 and automatic gives the Sportside muscular performance. But if you want a less powerful engine or one that can pull a massive load, Chevy has all the bases covered. No less than five engines - ranging from a 165-horsepower, 4.3-liter V-6 to a 230-horsepower, 7.4-liter V-8 - are available as well as two cargo-hauling diesels. If you would use a truck as a car - that is, to drive to and from work everyday and occasionally haul heavy loads - the 5.7-liter V-8 and automatic might be the best set up. The V-6 doesn't give you many more miles per gallon, and it can't tow as much. You'll also appreciate the 5.7-liter's ample strength when you are passing slow-moving traffic. Ourtest truck delivered fuel mileage slightly better than its EPA-rated 14 miles per gallon in the city and 19 on the highway. Using the air conditioner, the test truck returned 15 mpg in the city and 21on the highway. HANDLING Chevy's Sportside is one of the best-handling full-size pickups money can buy. Take a tight curve at, say 40 mph, and you'll feel the Sportside's firm suspension system keep the cab straight and the wheels planted firmly on the pavement. You won't hear any squealing of tires or feel the truck's weight shift. Even in inclement weather the Sportside's ability to hold the road is nothing short of astonishing. Stellar chassis and suspension work by Chevy's engineers, the truck's wide stance and its fat tires put the Sportside in a rare class. When I test a truck, I drive it repeatedly over a variety of bad roads - usually poorly maintained dirt roads - to see how well it handles deep potholes in qu ick succession and other types of lousy terrain. If any of the trim pieces are loose or if the suspension is not up to the task, this is where I'll usually find out. I haven't driven a pickup truck that's better than the Sportside in isolating the cab from the work of the suspension system. Over even the worst bumps, the cab dips gently while the front independent and live axle rear suspension system absorbs the shock. On the road, the Sportside drives as smooth and quiet as a mid-size sedan. The power-assisted steering is sharp and responsive, and the brakes are strong and powerful, though the antilock feature is active only on the rear brakes. FIT AND FINISH The test truck sported a glossy bright-red paint job that looked fine from a distance. But a closer examination showed that in some areas, the paint's smoothness was marred by what auto body repairmen call ''orange peel.'' Aside from that minor gripe, the test truck was flawlessly assembled. Inside, two cloth-covered bucket seats are separated by a big console with a built-in c up holder. The console isn't just for show. It offers real storage room - it was a handy place to stow rented videotapes. Behind the driver's seat on the floor there's a small tray designed to hold items such as an umbrella. It takes awhile to get used to the Chevy's dash. Unlike Ford's F150, the Chevy does not have a sweeping one-piece carlike dash. As it is now, the dash requires the driver to lean too far forward to work the cassette. The steering wheel seems to poke out a little too far, and the nicely styled instruments are housed in a large box attached to the dash. It's functional, but not attractive. There's enough headroom to wear a 10-gallon hat. Visibility is good, and the switches for the power accessories are easy to use, mounted as they are on the door panels. For its $20,000 asking price, I would expect power mirrors. That was the only glaring miscue equipment-wise. All in all, the Chevy Sportside Pickup is a well-built comfortable and rugged truck with excellent performance. It's a vehicle you would be proud to own. Truett's tip: Chevy's Sportside Pickup is a brutally handsome, powerful and well-engineered truck. It's classy enough to use in place of a car.