2008 Chevrolet Tahoe Hybrid 4x4
GM's full-size hybrid is the real deal. But is it better than a diesel?
This is GM's self-proclaimed "no-excuses hybrid." You can have it all: a gargantuan SUV with seating for eight, a big 6.0-liter V-8, towing, hauling, four-wheel drive, and fuel economy. That's right, GM's first "full" hybrid boosts the four-wheel-drive Chevrolet Tahoe/GMC Yukon's fuel economy by a whopping 43 percent on the EPA city cycle and five percent on the highway cycle to a V-6 Toyota Camry-shaming 20-mpg city rating, as well as a 20-mpg highway rating (21 city and 22 highway on two-wheel-drive models).
As part of this "no excuses" business, GM is proud that, unlike some single-purpose hybrids (think two-seater Honda Insight), the Tahoe maintains all existing functionality and features. Mark this one down: The Tahoe is the first hybrid to come with keyless starting; heck, it's even standard. That's right, Mr. and Mrs. Overindulgent American Greenie, the press of a button fires up six liters of gas-guzzling V-8 goodness in the driveway. You may be worried about your carbon footprint, but a cold leather seat is no way to start the morning.
What, you thought Toyota would think of this first?
Making this fuel-economy leap possible is a host of changes, but the most interesting is GM's patented two-mode hybrid transmission that packages in the same space as one of the company's venerable four-speed automatics. What sets this system apart from the popular Toyota hybrid arrangement is that the Tahoe can run as a continuously variable transmission (CVT) and an automatic through one of four fixed gear ratios. With their theoretically infinite ratios, CVTs are much better than conventional automatics at keeping engines spinning at the most efficient speeds, but at constant highway speed, that advantage can be nullified by the internal friction inherent to CVT operation. In hybrids, the energy loss can be even worse because the drivetrain must also spin an electric motor. The Tahoe, however, can switch over to a more-energy-efficient mechanical path (one of the fixed ratios in the automatic) to minimize wasted energy whenever possible. Whereas the Prius is always processing its power through the CVT, even at inefficient highway speeds, the Tahoe's system is better and can simply bypass the electric motors and lock onto a gear.
The power comes from GM's familiar pushrod 6.0-liter V-8--chosen instead of the 5.3-liter, GM says, because the larger engine is able to run in four-cylinder mode more of the time--as well as two 80-hp, 12,000-rpm AC electric motors fed by a 300-volt Panasonic DC battery pack (1.8 kW/h) housed under the second row. However, to further boost efficiency, the hybrid's small-block V-8 uses a late-closing intake-valve tuning that sacrifices some low-end torque as well as top-end power. That's why the Tahoe's output is 332 horsepower and 367 pound-feet of torque, down from its typical 366 horses and 380 pound-feet. Of course, the electric motors are there to fill in at the low end, even though they don't add anything to the peak power output of the overall system.
A Host of Tweaks
All the hybrid hardware adds weight--about 350 pounds in the Tahoe's case, GM says--which is not the direction to go to maximize efficiency of an already porky SUV. To counteract this and admirably maintain a 5850-pound weight (5650 for two-wheel-drive models), the Tahoe changes a number of pieces to aluminum: the hood, the front-bumper beam, the driveshaft, and the rear liftgate. It also gets thinner, lighter seats and forged aluminum wheels. Even the recommended tire pressures are bumped up by 2 psi to eke out every last bit of efficiency.
To maximize aerodynamics--trimming the coefficient of drag from 0.36 to 0.34--the hybrid's lower front fascia is devoid of holes and extends closer to the ground to decrease the amount of turbulent air passing beneath the truck, and a roof-mounted spoiler at the rear smoothes airflow over the top. Four-wheel-drive models are lowered 0.4 inch in the front.
It Drives Like, Well, a Quirky Tahoe
The Tahoe can propel itself on electric-only power (up to 32 mph) and, during our driving time, did so quite often. As we'd pull into a subdivision, for example, the Tahoe would shut down the V-8 and stay in electric mode for a half-mile or more as we wound our way through the low-speed streets. The gas engine also shuts down at stoplights, but pulling away at a rate that won't annoy those behind you will make it fire back up almost immediately.
GM says the hybrid can run its V-8 in four-cylinder mode at speeds up to 75 mph, although we can't verify that claim. We almost never saw the four-cylinder indicator--it resides in the trip computer--at anything near highway speeds, and even at much lower speeds, any slight grade would quickly refire the four dormant cylinders.
Driving normally, we managed a respectable 18 mpg over a 250-mile weekend, far better than the 12 mpg we got in a standard 5.3-liter V-8 Tahoe. And the hybrid's acceleration is comparable to that of the 5.3-liter V-8 as well, charging to 60 mph in just over eight seconds. But the power delivery, now that's different. At the low end, the hybrid feels a bit weak, but it comes on with a surge of power in the midrange. And there's still that familiar drone that accompanies many vehicles using CVTs to keep their engines revving at high, constant rpm for an extended period under hard acceleration.
Using the same 17-inch tires from other Tahoes, the hybrid actually has better stopping distances, GM says, because the anti-lock braking system was able to be optimized for a single tire. The feel of this electromechanical brake system that captures energy to charge the battery under deceleration, however, is lacking, and smoothly braking at your desired rate involves much guesswork. Trying to brake at the threshold of ABS activity proved nearly impossible because, at a predetermined point in the pedal's travel, the system goes to full-on panic-stop mode. But, then, how many Tahoe buyers even know what threshold braking is?
Ride and handing felt comparable that of regular Tahoes--fairly agile for a body-on-frame, solid-rear-axle SUV--but the electric power steering has even less feel than the regular Tahoe's.
GM has yet to officially announce pricing, but the hybrid won't be the most expensive Tahoe, the company claims. Since a fully loaded Tahoe LTZ comes in comfortably above $50,000, we expect the hybrid to start at about $48,000 when they hit showrooms shortly.
Does It Make Sense?
The Tahoe/Yukon is only the start of GM's two-mode-hybrid plan. GM has already forged partnerships with Chrysler and BMW, so expect to see Dodge Durango/Chrysler Aspen hybrids as well as the X6 hybrid in the near future. In the GM fold, the Cadillac Escalade hybrid is next, followed by this system in its full-size Chevrolet Silverado/GMC Sierra pickup trucks.
And the system certainly works, producing substantial fuel-economy gains. But during our time with the Tahoe, we couldn't ignore this burning question: Why not just have a diesel Tahoe instead?
Consider this: The similarly sized, seven-passenger Mercedes-Benz GL320 is powered by a 215-hp, 3.0-liter V-6 turbo-diesel with 398 pound-feet of torque. That 5 Best Trucks-winning SUV gets an EPA rating of 18 city and 24 highway and handily out-thrifted the Tahoe by 20 percent in our hands, netting 22 mpg over a similar driving regime. Despite having a far better seating package, the GL320 suffers from none of the Tahoe's quirks (numb steering, lack of brake feel, and abnormal power delivery) and actually tows more (7500 pounds versus 6000 for the Tahoe). Yes, starting at $53,775, the GL320 costs more than the Tahoe hybrid. However, the GL costs less than an Escalade and will likely be less expensive and more fuel efficient than the '09 Escalade hybrid.
For sure, both clean-diesel and hybrid technologies are extremely expensive to develop. But we imagine that once a diesel engine is developed, it is likely far easier to install in multiple vehicles (as Mercedes has done, putting this diesel in everything from the GL-, R-, and M-class SUVs to the E-class sedan) without painstakingly retuning the regenerative brakes, power delivery, and packaging; and without all the added complexity and relatively unknown service life of the electric motors and battery pack.
In light of all this, it seems as though hybrids are definitely still fighting an uphill battle to prove themselves as the near-term fuel-efficiency technology of choice.