ROAD TEST

Their availability in hybrid version this year is no reason in itself to buy a sport utility vehicle as large and heavy as a Chevy Tahoe or GMC Yukon. About 20 mpg probably is the best you'll get out of it in the real world.

But if you need that kind of interior space with three rows of seating or 109 cubic feet of cargo room - or 6,000 pounds of towing capacity for your boat or camper - the hybrid powertrain will take some of the pain out of fill-ups. And best of all, this remarkable hybrid exacts almost no compromises for driver or passengers.

Its operation is nearly seamless; except for a slight shudder as the engine automatically restarts on acceleration from a stop - and some whine that I assume is from the electric motor - these hybrids drive like their conventionally powered brethren. The hybrid versions went on sale in January. Hybrid variants of the Chevy/GMC Silverado/Sierra full-size pickup trucks are due in showrooms late this year.

If guilt is all that's keeping you from buying such a big SUV as rising gasoline prices bring home to us the true cost of our evil energy ways, the hard-to-miss "Hybrid" decals on the testers' flanks and windshield will let the neighbors know you cared enough to buy the very best - at least for your wallet, if not about global warming.

The only catch in opting for the hybrid is a higher price upfront - over a comparably equipped conventional Tahoe or Yukon. How much higher, though, is unclear and depends on how you would have equipped your conventional Tahoe or Yukon if you had opted for one. If GM has an estimate of the average premium one pays for hybrid models, it's not sharing it with us.

The hybrid models have a number of specific features that are difficult for an outsider to evaluate in dollars and cents. The hybrids' front and rear fascias, wheel openings, running boards, rear spoiler, tail lamps, and certain other elements of the body are modified to improve mileage further by reducing wind turbulence and drag.

A slightly larger grille allows increased airflow to the radiator and engine compartment, helping to cool the hybrid power electronics. Engineers also tried to reduce weight where possible, such as substituting aluminum hoods and liftgates for steel. Those cost extra.

On the other hand, one assumes GM saved some money by eliminating the spare tire, jack and other tools in the hybrids to save weight, replacing them with a tire inflator kit.

The hybrid versions come just one way - loaded. So while Tahoes and Yukons can be had beginning in the mid-$30,000s, the hybrids start at $50,490 for the Tahoe and $50,945 for the Yukon, both including freight.

Then there's a federal tax credit of up to $2,200 that will reduce the hybrid's price for taxpayers who qualify for it. So what's the gain in fuel economy?

At the risk of boring you with numbers, the best you can do in a conventional Tahoe is 14 mpg in city driving and 20 on the highway, says the EPA - in a Tahoe with a 320-hp. 5.3-liter V-8 and two-wheel drive. The hybrid, whose 6-liter V-8 gasoline engine and electric motors deliver a total of 332 hp., is EPA-estimated at 21 mpg in the city and 22 on the highway - in two-wheel drive. Add four-wheel drive, as most Northerners probably will (and which my Yukon tester had), and the highway estimate for the conventional model drops by 1 mpg; the hybrid model loses 1 mpg in the city, 2 on the highway. In either case, it's clear that the EPA is forecasting the biggest gain from the hybrid powertrain in local driving, when it can spend at least some of its time creeping along in heavy traffic on the electric motor alone. The engine shuts off whenever the vehicle stops, and starts when it's needed to accelerate.

A GM spokesman says the engine also will shut down at times in steady-state cruising, but I never saw that happen in my week with the two hybrids. And, in my experience, any driver with a foot heavier than a sparrow's will almost always need the gasoline engine to accelerate in local suburban driving.

According to the testers' driving computers, my trip averages, with about 75 percent of the miles on the highway, were between 18 and 21 mpg. (I noticed a small improvement when I switched the Yukon's four-wheel-drive system off and ran in two-wheel drive, just as a GM spokesman predicted.)

So what's saved in real money? The EPA says a 4WD hybrid Tahoe will burn $2,235 worth of gas per year, assuming it is driven 45 percent on the highway and 55 percent locally for 15,000 miles a year, and assuming gasoline costs $2.98 a gallon.)

The 5.3-liter conventional Tahoe will cost $2,794 to operate, says the EPA. Savings, then: $559 a year, or 20 percent - a little less than the 25 to 30 percent fuel economy improvement that GM claims - at least as calculated by the EPA - but not too shabby nonetheless.

And eventually, the hybrid buyer can expect to recoup the higher purchase price in fuel saving.

Naturally, the higher the price of gas, the shorter the payoff time; $2.98 a gallon is a fading memory in the New York metro area. "Regular" averages more than $3.30 a gallon on Long Island, and it's still climbing. No matter what powers them, the Tahoe and Yukon are the best full-size SUVs GM has ever offered and, for the money, maybe the best available. As most of you probably know, they're produced also in extended version, called Suburban and Yukon XL, but those versions aren't available with the hybrid powertrain.

GM has worked hard to make these behemoths easier to drive and comfortable to ride in, but the non-independent rear suspension still is apt to deliver a rude jolt to passengers and even do a slight sidestep on a bump. The electric power steering - specific to hybrids - keeps the driver in just minimal contact with the road. There's signficant body lean in cornering. A lot of brake pedal pressure is needed to stop these nearly 6,000-pound bruisers. And the blind spots next to them on the road are big enough to hide two Minis or four Smarts.

On the upside, the interiors of both the Chevrolet and GMC testers were beautifully trimmed. Gauges in the clusters in front of the driver included voltage and oil pressure indicators - two items fast disappearing from modern cars - and the entire cluster is clearly readable, day or night. Controls are self explanatory, conventionally located and easy for the driver to reach. The tach, unique to the hybrid versions, includes an "auto stop" indicator to tell the driver when these very quiet and smooth-running gasoline engines are off. It's often hard to tell without the gauge.

A fuel economy gauge located to the left of the tachometer helps the driver maintain the most efficient driving style. Located midway down the center stack, the standard navigation system's 6.5-inch full-color display doubles as a monitor that shows a representation of the hybrid drivetrain's power flow.