Going car-shopping with my sister and brother-in-law recently as they sought to replace their aging full-size SUV, I found out exactly why General Motors’ new line of large crossover utility vehicles is selling so well.

Those three vehicles, the Saturn Outlook, GMC Acadia and Buick Enclave — built on the same architecture with similar features – are selling so well that dealers have a hard time keeping them in stock.

Even though my sister and her husband ended up buying the Acadia, I believe they would have been just as satisfied with the Outlook or our test vehicle for this week, the 2008 Enclave.

The differences among the three are so slight that anyone looking for a roomy crossover with great styling, decent fuel economy and lots of interior space should be happy with any of them.

Making the switch from a truck-based full-size SUV from a different manufacturer was a big step for my brother-in-law, who is a longtime friend of his hometown dealer and has been buying cars only from him for years.

He was so concerned about his decision to go with a different brand (and dealership) that he even called his dealer friend before he went to take delivery of his new Acadia, apologizing for his decision.

The decision wasn’t a hard one for him otherwise, however. His friend’s dealership just didn’t have a vehicle that he felt met his needs at this particular time.

With gasoline prices skyrocketing, he wanted a vehicle with better fuel economy. Also, the big SUV’s size was a problem for my sister, who never liked driving it because it was so big. She also never cared for the rough ride, a result of the vehicle being built on the architecture of a pickup truck.

My brother-in-law’s dealer friend brought him a midsize crossover to drive for a weekend recently. They liked its styling and fuel economy, but the vehicle just didn’t have the space they wanted.

With grandkids that they often haul around, they need a third row of seating frequently. But the midsize they tested didn’t offer a third row.

My sister and brother-in-law were interested in one of the new Chevrolet Tahoe or GMC Yukon gasoline-electric hybrids, but didn’t want to wait two or three months to get one.

At a Buick-Pontiac-GMC dealer, they found the Acadia, which turned out to be exactly what they were looking for: a roomy family vehicle with three rows of seats, carlike ride and handling, decent power, and good fuel economy.

EPA ratings for all three of these vehicles are 16 miles per gallon city/24 highway (using the more-stringent 2008 EPA rating formula) for the front-drive models, and 16/22 for the all-wheel-drive versions.

This column actually is about the Enclave, though, not the Acadia.

My sister and brother-in-law’s shopping experience is germane to this review, though, because the vehicles are so similar.

They chose the Acadia over the Enclave because that’s what their dealer had the most of to choose from – including one with a maroon exterior color my brother-in-law wanted to show his support for Virginia Tech, his alma mater.

The Enclave would have suited him just as well, had their been one on the dealer’s lot with the right equipment (and his favorite team’s color).

Their shopping experience was a real eye-opener for me, though. It made me realize just what an advantage GM has right now with these large crossovers.

With a carlike chassis that combines frame and body in one unit, they are lighter and easier to drive than vehicles such as the Expedition, whose trucklike construction includes a body on top of a steel frame.

The weight saved by combining body and frame into a unibody format, the major difference between a crossover and a traditional SUV, is one reason the crossovers get better mileage.

The Enclave/Acadia/Outlook are built on a chassis GM developed for what was to be its next generation of minivans, although the automaker has since abandoned plans for any new minivans.

The Enclave has a starting price of $32,790 (including $735 freight) for the base, front-drive CX model and $36,990 for the uplevel, all-wheel-drive CXL, our test vehicle. It replaced two SUVs in the Buick lineup. They were the car-based Rendezvous and the truck-based Rainier.

The Rendezvous, like the Enclave, had minivan roots – it was based on the current GM minivan architecture that also spawned the now-discontinued Buick Terraza minivan.

GM plans to abandon the traditional minivan and midsize truck-based SUV segments, replacing both with these new crossover models.

A fourth version, the Chevrolet Traverse, will arrive next year as a 2009 model. For Chevrolet, that will mean the end of the TrailBlazer SUV and Uplander van, for Buick, the Rainier and Terraza (both already gone), and for GMC, the Envoy. Major competitors of the Enclave are the Acura MDX, built on the chassis of the Honda Odyssey minivan; the MKX; the Lexus RX 350; and the Volvo XC90. But the Enclave is the roomiest of the bunch.

Styling is another of the ways that crossovers have differentiated themselves from traditional SUVs, moving away from the boxy looks of the truck-style SUVs. That was another of the selling points of the Acadia for my sister and brother-in-law.

The Enclave and its siblings have sleek exteriors that were developed with the help of wind-tunnel testing. Their aerodynamic shapes also help with fuel economy.

Under the hood of the Enclave (and the others) is a 3.6-liter V-6 engine that generates 275 horsepower and 251 foot-pounds of torque, which puts it among the leaders in the midsize premium crossover segment. It’s connected to a six-speed automatic transmission, common among the newest midsize crossovers.

The Enclave’s interior is quieter than that of the MDX, XC90 and RX350 at highway speeds, and that it has segment-leading cargo space behind the third seat. Surprisingly, the third seat is even comfortable for adults. With the optional captain’s chairs in the middle row, there is a walk-through area between them that allows kids easy access to the third row.

With a bench seat in the middle, the Enclave can carry eight people; with the middle-row buckets, seven.

A center console armrest between the front bucket seats can slide forward or backward to accommodate the driver and front passenger’s arms.

Standard features on our Enclave CXL all-wheel-drive model included a power tailgate, OnStar with turn-by-turn navigation, electronic stability control, side-curtain air bags for all three rows of seats, tilt-and-telescopic steering wheel, XM satellite radio, power everything, antilock brakes, seat mounted front side air bags, and even a tire-pressure monitoring system.

Also, our vehicle came with a leather-wrapped steering wheel with wood accents; three-zone climate control; ultrasonic rear parking assist; heated windshield-washer fluid; and leather seating surfaces. The front seats have built-in heaters.

Options included a dual panoramic sunroof ($1,300) for front and second-row passengers; an entertainment/navigation package ($3,025), which brought a Bose sound system and touch-screen navigation system with backup camera; 19-inch chromed wheels ($1,495); and a luxury package ($925), which added heated outside mirrors, active headlights that turn the direction the car is turning, and a power tilt and telescopic steering column.

Red-jewel exterior paint was $395 extra.

A rear-seat DVD system is available, although it was not included on our tester. Total price of our vehicle was $43,530, including freight, options, and a $600 credit for the original wheels.

G. Chambers Williams III is staff automotive columnist for the San Antonio Express-News and former transportation writer for the Star-Telegram. Contact him at chambers@star-telegram.com.

At a Glance: 2008 Buick Enclave

The package: Large, five-door, seven- or eight-passenger, front- or all-wheel-drive crossover utility vehicle.

Highlights: This is one of three new large GM crossovers built on the same architecture. The others are the Saturn Outlook and Buick Enclave; a Chevrolet version, the Traverse, will arrive for 2009. They are quite refined, have lots of power yet decent fuel economy, and come with a variety of standard and optional features.

Disadvantages: No serious four-wheel-drive system offered for off-road use; no V-8 engine upgrade for better trailer-towing capability.

Engine: 3.6-liter V-6.

Transmission: Six-speed automatic.

Brakes, front/rear: Disc/disc, antilock.

Power/torque: 275 HP./251 foot-pounds.

Brakes, front/rear: Disc/disc, antilock.

Electronic stability control: Standard.

Side air bags: Front seat-mounted; side-curtain, all three rows, standard.

Length: 201.1 inches.

Curb weight range: 4,720-4,925 pounds.

Cargo volume: 19.7 cubic feet (behind third seat); 68.9 cubic feet (third row folded).

Trailer-towing capacity: 4,500 pounds.

Fuel capacity/type: 22 gallons/unleaded regular.

EPA fuel economy: 16 city/24 highway (2WD); 16/22 (AWD).

Major competitors: Lexus RX 350, Infiniti FX, Acura MDX, Lincoln MKX, Volvo XC90.

Base price range: $32,790-$36,990 including $735 freight.

Price as tested: $44,130, including freight and options (CXL model with all-wheel drive).

On the Road rating: 9.5 (of a possible 10).