Driven: 2008 Chevrolet Tahoe Hybrid, fuel savings in extra large

September 1, 3:29 PMAuto Review ExaminerJohn Matras

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A twenty miles per gallon Chevrolet Tahoe. It sounds like an oxymoron but it isn’t. Twenty miles per gallon is the EPA mileage rating for this 2008 Chevrolet Tahoe Hybrid with four wheel drive: That’s twenty miles per gallon city and twenty miles per gallon highway. What’s more, that’s just about what we achieved. Our overall mileage for a week of driving came to nineteen miles per gallon, and that’s without taking any special measures to stretch how far we could go on a gallon of gas.

Wanna know something else? The rear-wheel drive Tahoe Hybrid, the EPA says, gets twenty-one miles per gallon.
How does it do that? Magic. No, not really, but close. The engineers behind this system, which was developed as a joint program with Chrysler, looked at the various operating modes of a vehicle and combined various fuel saving and power-maximizing techniques to provide, surprisingly enough, maximum power and efficiency.

Watt’s My Line Here’s how it works. There are two 60 kW electric motors contained within an otherwise conventional four-speed automatic transmission. There’s a 332-horsepower 6.0-liter V-8 engine with Active Fuel Management, what General Motors calls the shut-down of four cylinders out of eight during light duty operation. And of course extra battery power, a 300-volt nickel-metal hydride unit which in GM-speak is ESS, for “energy storage system.”

What happens in normal operation is the Tahoe—it’s also available in the Tahoe’s corporate twin, the GMC Yukon—rolls away from a standing start using only one electric motor. If more torque is needed, the second kicks in…all seamlessly, of course…and then when more power is needed…at speeds of, say, twenty miles per hour or faster acceleration at lower speeds, the V-8 engine starts up and adds its power. The electric motors operate in conjunction with the automatic transmission, which means the engine rpm is fairly consistent around town.

Under higher demand, the hybrid system will shift to a direct connection through the transmission without the electric motors adding power, or if more power is needed for towing or acceleration, the motors can provide the equivalent of 30 horsepower. The electric motors can be used as generators to recharge the ESS.

As an added bonus, the electric motors can allow the engine to operate in four-cylinder mode longer for even more fuel saving.
Somehow General Motors defines the power-train as a two-mode hybrid. Seems like a whole lot more modes to us, but we’re not going to argue.

The Chevrolet Tahoe Hybrid’s operation isn’t wholly transparent. Twist the key and there’s a brief pause before, instead of the traditional starter motor’s whirring sound, the engine comes to life with a distinctive “schoooo” sound. The engine will continue to run until it reaches operating temperature, at which time it will go into AutoStop mode when the truck is stopped or when the controller decides the Tahoe can operate by battery/electric motor alone, usually under twenty-five miles per hour with light load. The tachometer needle will drop to “AutoStop”—halfway between 1000 rpm and full zero.

Days of whine and Tahoes A faint whine of the electric motors can be heard during light acceleration at a low speed. There’s a light thump when the control system tells the gas engine it’s time to start. It feels more like a torque reaction as the engine rocks on its engine mounts. Only the tachometer dropping to AutoStop mode, as when creeping through a parking lot, gives an indication that the engine has stopped running. Only then is it noticeable that the Tahoe is gumshoeing along. It’s likely that all mall driving will be done in stealth mode. We think the theme from Jaws should be played as a warning for pedestrians.

Romp on the gas pedal, however, for the full force of the 6.0-liter’s 332 horses and 30 horsepower equivalents (athletic cows, perhaps?). Better yet, despite a compression ratio of 10.8:1, the Tahoe Hybrid is happy on regular 87-octane gas. And better even yet, our test four-wheel drive Tahoe has a tow-rating of six-thousand pounds, the rear-wheel drive Tahoe Hybrid can pull up to sixty-two hundred pounds. If hybrid has ever meant “puny”—and we can think of more than a couple of examples—it’s an adjective that doesn’t apply to this SUV.

There are a few internal visual cues to the Tahoe Hybrid’s hybridity. The tachometer as mentioned has an AutoStop position, and there’s an economy meter but the needle of the latter only seems to go from vertical to right, over to the bad zone.

 The screen for the navigation system and audio also can show a schematic of the hybrid system at work, though simplified even more than our description here. It’s at least as entertaining as a similar display on other hybrids.

Extra large for the price of a large The Chevrolet Tahoe two-mode hybrid is really a remarkable vehicle, getting sedan mileage from a seven-passenger full-size SUV and still being able to do anything a full-size body-on-frame 5,800 lb SUV can do. Which is a lot more than can be done with a Prius. Next up for the two-mode hybrid is the two-wheel and four-wheel drive Dodge Durango and the front-drive Saturn Vue.

Obsolete and bordering on extinction? Not these SUVs. America short on technical expertise? Don’t think so. The only drawback right now is price: Our test vehicle has a base price of $52,395, but with options including the rear seat entertainment system ($1,295), and sunroof ($995), it had a list price of $54,685 (after $900 destination fee). Every hybrid costs more than its less-sophisticated alternative but for this…nurse, get the paddles. It’s the big one…

Nevertheless, the Chevrolet Tahoe Two-mode Hybrid is a very remarkable piece of equipment, and without a doubt the most advanced hybrid vehicle on the road today. No wonder Chevy puts so many hybrid labels all over the outside. It’s a wonder none of them say “20 mpg SUV.” 

Illustrations, top to bottom: 2008 Chevrolet Tahoe Hybrid: exterior, hybrid controller, dash, hybrid operation display. Photos courtesy General Motors.