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.