Not all big SUVs have to get bad gas mileage, according to General Motors. I guess it depends on what you consider bad.

The General is rolling out its latest hybrid system--its first full hybrids--on full-sized three-ton SUVs this fall. The automaker estimates the new Chevrolet Tahoe and GMC Yukon "Two-Mode" hybrid SUVs will get about 20 mpg, city or highway. Indeed, driving around New York city last month, making no attempt at maximizing fuel economy, the trip computer showed 19 mpg. Feathering the gas to maximize driving on the batteries brought that up to an indicated 24 mpg. Our sophisticated fuel economy testing has shown differences between real-world MPG and what trip computers project. Basically, we'll believe it when we see it (or, in this case, test it).

That's not bad for a full-sized SUV, but it's worse than any hybrid we've tested before. When you consider the performance, 20 mpg starts sounding better, though. These are the first hybrids with full-sized towing ability (6,800 pounds--a little less than a typical Tahoe's 7,200-pounds). And GM says they can accelerate from 0-60 in about 8 seconds.

All this is accomplished by GM's new full-hybrid system that uses two electric motors inside the transmission to allow the trucks to shut off and restart the gas engine at stoplights, accelerate from a stop silently on electric power, recharge the batteries while braking, and use the electric power to assist the engine at higher speeds. Like most hybrids, the CVT (continuously-variable transmission) has an infinite range of gear ratios. For heavy-duty work the transmission shifts into a traditional direct drive mode with four stepped gears just like a traditional four-speed automatic. The hybrid system adds 400 pounds to the Tahoe's curb weight. Partially offset by other weight savings, that pushes the Tahoe hybrid's total to about 6,000 pounds.

Unlike other Tahoes, which are mostly powered by a 5.3-liter V8,the two-mode hybrids use GM's larger 6.0-liter gasoline V8 with cylinder deactivation, which shuts off four cylinders when their power isn't needed (this feature is also on the 5.3). The biggest fuel economy benefit on the highway comes from the electric motor keeping the V8 engine operating on four cylinders more than other Tahoes. In other GM trucks and SUVs we've driven with this technology, it's very difficult to keep the engines in four-cylinder mode except when going downhill or decelerating. In the two-mode hybrid, we were able to cruise most of 100 blocks down New York's West Side Highway at 60 miles per hour on four cylinders.

The price premium for the hybrid hasn't been announced. No GMC Yukon Denali or top-level Tahoe LTZ will be available.

If the 20 mpg figure ends up being comparable to our 14 mpg our non-hybrid test Tahoe returned, it will represent a huge improvement. However, most minivans can return similar mileage overall and better on the highway, ride better, and have more space inside. The only real advantage is for buyers who need towing ability and want better mileage than a typical full-sized SUV during most of their driving when they're not towing.

I wonder how many buyers interested in paying a premium to be green will get a thrill from the acceleration. Does the world really need another 6,000 pound SUV that can hit 60 mph in 8 seconds, hybrid or otherwise? And is that what hybrid buyers want?

Post your comments below. We're interested in your take.

--Eric Evarts