Unless you're a GM aficionado, it would be easy to miss the fact that a Hybrid Tahoe is in fact a hybrid, at first glance. No doubt, that's exactly what Chevrolet was planning. Appearance-wise there's really not much difference, aside from the badging and additional hybrid graphics that adorned the press vehicle we tested. The grille is a bit more tapered at the bottom, and the bumper cover is more flush with the grille. C-pillar plastics provide a more streamlined path for air, but are hard to notice. It is practically the same interior that we've grown to love in our Silverado LTZ.
The real difference comes when you first fire up the 6.0L engine. (Yes, the hybrid uses a larger engine than the standard Tahoe, more about that in a minute.) The typical sound of a starter engaging the flexplate is gone, replaced by a muted growl as the engine fires up from the electric motor attached to its crankshaft, where a torque converter would normally be. It's even more noticeable when using the remote start, because from inside the cabin, the engine noise at idle is hardly audible. In fact, throughout the engine's operating range, it seems quieter.
The 2-Mode hybrid system in the Tahoe was a joint engineering project between BMW, Daimler-Chrysler, and GM, and uses packaging dimensions similar to traditional automatic trans-missions. We had GM engineers explain the intricacies of the system in person, with a cut-away 2-mode system to help us out, and it's still complicated, but here's the shortened version.
The planetary gears are still there, but rather than a torque converter, there are two electric engines that can either generate power or propel the vehicle down the road. The difference between engine speed and transmission speed is absorbed here, much like a torque converter, but rather than excess energy heating transmission fluid, it can be used to generate power. This happens while the engine warms up and the vehicle isn't moving.
Once the engine reaches operating temperature, the engine shuts down as the vehicle comes to a stop, around 12 mph, and doesn't come back on until the electric motors accelerate the vehicle from 12 to 15 mph. At that point, both the electric motors and the engine power the Tahoe, with the engine taking over at highway speeds.
To get extra efficiency out of the 6.0L engine, it uses a late-closing intake valve to create an Atkinson cycle. Essentially, the dynamic compression ratio is lowered because cylinder pressure is bled off when the intake valve is open and the piston is traveling up. When the air/fuel ignites, it expands to a larger volume than it was compressed from, which is more efficient. Additional fuel-saving methods were also employed. The bump in displacement over the standard 5.3L Tahoe drivetrain allows the 6.0L to run in four-cylinder mode longer, and at higher vehicle speed than a similar 5.3L.
All of this trouble for a fullsize SUV might make you ask, "Does a fullsize hybrid SUV make sense?" Let's take a look at the fuel mileage from our long-term test Silverado for comparison. After plenty of time to get broken-in, it delivers 14.8 mpg. Granted, it's a 4x4 and the Tahoe is 2WD, but it's still a pretty fair comparison. The 2-Mode Tahoe we drove got 20.1 mpg and was still climbing up, gradually, when we had to turn it in. Undoubtedly it would continue to climb up at least a few tenths, especially if you had an eye on economy, which we didn't. That's still a 36-percent improvement. With current gas prices, that would save the average driver about $1,000 a year. GM hasn't given a hard cost difference between hybrid and normal Tahoe models, but when optioned the same, the option seems to cost somewhere in the neighborhood of $8,000. So, like most hybrids out there, it's going to take a while to recoup your investment through fuel savings. That's the downside. On the plus side, the 2-Mode system was so seamlessly integrated into the Tahoe that you'll hardly notice it, except at the gas pump.
PRICE (as tested)
6.0L OHV eight-cylinder
322 at 5,100 rpm
367 at 4,100 rpm
Four-speed automatic (M99)
Independent with Coilover (front), five-link and Live Axle (rear)
Hydraulic-boosted four-wheel ABS, traction control, and stability control
20.1 (as tested)