Road Test: 2008 Chevrolet Tahoe Hybrid
This 2008 review is representative of model years 2008 to 2013.
By Csaba Csere of Car and Driver
Mention hybrid, and most people think of fuel misers, such as the 40-to-50-mpg Toyota Prius. But a hybrid powertrain can save fuel in vehicles of any size, as demonstrated by this Chevrolet Tahoe, which represents the first example (along with the GMC Yukon and 2009 Cadillac Escalade) of hybrid technology applied to a large SUV. As such, it gets a big-time fuel-economy bump, but as a 5840-pound vehicle, it isn’t—and can’t be—a fuel miser by anyone’s definition.
This Tahoe is the first “full hybrid” from General Motors, meaning that it can run purely on electric power at low speeds. It uses a new two-mode hybrid system that is a joint development of GM, BMW, Chrysler, and Daimler and is fundamentally different from the hybrid powertrains used by Toyota and Honda.
The transmission resembles a conventional four-speed automatic to which two 80-hp electric motors are integrated by marrying them to the transmission’s three planetary gearsets. Despite the two-mode name, the transmission, which fits into the same space as the automatic in conventional Tahoes, seems to operate in three distinct modes.
The first mode is continuously variable (CV) and provides ratios from infinity to 1.70. The Tahoe always starts in this mode because it lets the transmission take off smoothly from a stop without the need for a torque converter. When accelerating gently, you can stay in this mode using only the electric motor up to 32 mph, but usually, the 6.0-liter V-8 cuts in to provide additional power.
The second mode also uses CV operation but with a different planetary gearset to provide ratios from 1.70 to a deep overdrive of 0.50 (used during coasting and regenerative braking). This mode provides the benefits of a CVT at higher speeds while keeping the electric motors running in their most efficient rpm range.
Finally, the transmission has four fixed ratios, which are selected when the electric motors are needed to charge the batteries or when towing heavy loads. The powertrain management system, which GM calls the “hybrid optimizing system,” juggles the mode and the ratio to provide the most efficient operation at each driving load.
The one downside of this Atkinson-cycle strategy is that it compromises low-rpm torque. That’s why the hybrid gets a 6.0- liter V-8 rather than the standard 5.3, which might have seemed the more obvious high-efficiency choice. Like the 5.3, however, the 6.0 also gets Active Fuel Management, which means the engine can shut off four cylinders and operate in a V-4 mode during light load conditions.
Powering the electric motors is a 288-volt, 6.5-amp-hour battery through a controller under the hood. This piece of heavy-duty circuitry also runs two auxiliary components: the A/C compressor and the power steering. When the V-8 shuts down, electricity keeps these devices operating.
The electric motors in the transmission operate as generators under braking to recharge the battery, up to 0.30 g of deceleration. Brake harder than that, and the conventional hydraulic brakes engage to slow down as quickly as tire traction permits.
The hybrid hardware adds a weight penalty of about 350 pounds, but that’s offset by lighter seats and wheels, a smaller 12-volt battery, no starter motor, and an aluminum hood, liftgate, front bumper beam, shaft, and engine. Sure enough, our Tahoe hybrid weighed 5840 pounds, exactly the same as the Tahoe LTZ that we tested two years ago, making this Tahoe the first hybrid we’ve seen that doesn’t weigh more than its conventional counterpart.
Like most other hybrids on the market, the Tahoe gets several changes designed to improve its overall efficiency. A deep front air dam, a rear spoiler, front and rear wheel spats, and reworked front and rear fascias, running boards, and D-pillar moldings help reduce the drag coefficient from 0.36 to 0.34.
How much efficiency does this hybrid magic provide? By EPA measures, the hybrid Tahoe four-by-four is rated at 20 mpg city and highway. The standard Tahoe four-by-four’s ratings are 14 city and 19 highway. Our as-tested mileage for the hybrid was 19 mpg, way better than the 12 mpg we got with the regular Tahoe, although both machines were driven on random, not necessarily comparable cycles.
We rounded up a GMC Yukon SLT four-by-four equipped with the 5.3-liter V-8, just like a base Tahoe, and drove a mixed urban-and-highway loop in convoy with the Tahoe hybrid to get a more direct mileage comparison. The loop averaged 43 mph, and the hybrid got 23 mpg compared with 18 for the Yukon. The Tahoe hybrid’s fuel economy improved by just five percent on the highway cycle, but by the other measures, it delivers 27-to-58-percent-better fuel efficiency.
That’s a useful improvement, given that the hybrid is a few 10ths quicker to 60 mph and through the quarter than the standard Tahoe. It also has a 6000-pound towing capacity and seating for eight, although the third-row seat is hardly attractive to anyone more than five feet tall.
The Tahoe hybrid driving experience will be transparent to most drivers. Although the engine shuts down almost every time you stop, the Tahoe moves off instantly under electric power as soon as you touch the accelerator, with the engine firing quietly soon after. The transmission shifts through its modes smoothly, and if you sometimes sense engine-rpm changes that are a tad unfamiliar, the experience is still much closer to a conventional automatic’s than to that of the mechanical CVT.
Drivers with elevated sensitivity will notice that the electric power steering is light on effort and even lighter on feel, but the steering is accurate, and positioning the hybrid requires no great attention. The hybrid’s brakes feel somewhat nonlinear, but it’s no problem to scrub speed smoothly.
Driving for optimal economy is achieved through light touches on the accelerator and brake, behavior encouraged with an inscrutable “economy” gauge. Some labeling would make this unit more comprehensible.
If you select “instant fuel economy” on the trip computer, you can see when the V-8 switches to four-cylinder mode. The hybrid’s electrical boost is used to increase the time spent in this mode, but it doesn’t take much of a grade to cause the engine to go to V-8 mode, even at moderate speeds. Still, with a roughly 10-mpg jump when the engine switches from V-8 to V-4, even occasional four-cylinder operation is helpful.
The Tahoe 4wd hybrid comes only in a well-optioned version for $53,295. That’s about five grand more than a comparably equipped regular Tahoe. If you drive mostly on the highway, you’ll never make up the extra cost in fuel savings. But if you drive 15,000 miles per year, mostly in the city, you’ll save more than 300 gallons of gasoline per year. At $3 per gallon, you’ll make up the extra hybrid cost in about five years. And that’s a quicker payback than you’ll see in a Prius or a Civic hybrid.