Test Drive: Going hybrid makes Tahoe a treat

— -- General Motors plans to start selling gasoline-electric hybrid versions of its full-size Chevrolet Tahoe and GMC Yukon SUVs this fall, hoping their improved fuel economy will attract mileage-minded buyers back to the potentially profitable big SUV market.

The hybrids will come only well-equipped. But GM gm says they won't be the most expensive in the $35,000-to-$55,000 Tahoe/Yukon line.

A few hours in a preproduction Tahoe hybrid around Northern Virginia, in traffic and on highways, was satisfying.

Tahoe was smooth as you please in stop-go because the electric motor, which handles low-speed driving, provides all its power the instant it begins to turn. Gasoline engines have to rev a bit. GM says the electric-only mode could take you to 32 mph under ideal conditions. But the test showed that accelerating in traffic means electric-only lasts only up to about 10 mph.

The hybrid tows less, 6,000 to 6,200 pounds vs. 7,500 to 8,200 on gasoline models. No tow test was scheduled, but a stop at the Hobby Hangar in Chantilly, Va., a store that sells remote-control cars, planes and boats, provided an opportunity. The folks there, beyond all reason and good sense, offered the store's trailer. It was empty and probably weighed 1,000 pounds, according to the consensus, but it at least hinted how the hybrid would tug.

Very well, it turns out, with the trailer behind and five guys aboard the Tahoe.

Towing or otherwise, under hard throttle the hybrid was quieter than normal Tahoes and felt as if it weren't gaining speed fast. But it was. About the time your rump said 50 mph, the speedometer showed 70. What seemed 35 or 40 mph was 50 mph or faster.

Typical of hybrids, Tahoe saves fuel by shutting off its gasoline engine at stoplights and when the electric motor is sufficient to power the truck. But also unfortunately typical, Tahoe's gasoline engine shuddered as it fired up and began contributing power. Expect the shakes to be gone in regular production models, says Mark Cieslak, chief engineer for GM's full-size trucks.

The transmission got hung up on full-throttle shifts, holding the engine at 5,500 rpm too long, followed by a falloff in power, before up-shifting. Also gone by production, Cieslak promises.

The test truck's trip computer showed about 18 miles per gallon, less than the 20 or so GM forecasts, but the few miles of light towing hurt a lot.

The hybrid number beats the 13-to-17 real-world mpg of a gasoline Tahoe (Test Drive, January 2006).

GM predicts 40% better mileage than the gasoline model in city driving, where most people spend time.

Hybrids qualify for income-tax credit. That hasn't been calculated or approved by the government yet.

Converting to a combination of gasoline and electric power to improve fuel economy has improved the vehicle overall. Here's how:

•Lighter components wash out some 300 pounds of the 400-pound weight penalty imposed by the hybrid system's two electric motors, big battery pack and associated bits. Thus, the Tahoe hybrid hood and tailgate are lightweight aluminum and easier to lift and lower than the steel items on non-hybrid trucks.

Reconfigured front seats not only weigh less, they open 1.6 inches (40 millimeters, to be precise) more legroom for second-row riders.

The hybrid's 6-liter V-8 is aluminum alloy instead of the heavier, iron 5.3-liter V-8s in gasoline-only Tahoes, lessening front-end weight and providing better-balanced handling. Still not what you call sporty, as a fast run down an offramp will demonstrate.

The 6-liter has nearly 4% more horsepower and 8% more torque than the 5.3-liter in gasoline-only Tahoes.

•Reshaping front and rear panels improves airflow for better highway mileage. The subtle changes, plus lowering the vehicle 0.4 of an inch (10 mm), which also helps airflow, make the handsome Tahoe look even better. Tucking and trimming and better-integrating the running boards for airflow improvements dramatically boost the eye appeal of the rocker-panel area. Even aesthetically enlightened souls who hate running boards will find it hard to complain.

•Small details get attention. Switching from conventional taillights to light-emitting diodes (LED) adds — GM swears — 0.1 to 0.2 miles per gallon, because LED take less energy to illuminate. And they look cool.

•Serendipitous synergies are unlocked. Regenerative braking (regen), part of any hybrid, delivers firmer-feeling brakes the truck has needed. It turns the motor into a generator as you slow the vehicle, pumping juice back into the battery pack. That way, the gasoline engine doesn't have to come on as soon, as often, just to recharge the batteries. GM credits regen for 40% of the hybrids' total fuel economy improvement.

Electric power steering avoids the fuel-using engine drag of conventional, belt-driven power steering and improves the truck's steering feel.

Redesigning the front seats all but eliminated the overdone lumbar bulge that ruins many GM seats.

The automaker says it might eventually use some of the weight-saving measures on non-hybrid models.

If the preproduction tester is a true harbinger, and if hybrid prices aren't absurdly high, then GM is doing a great favor to buyers who need the beef and brawn of a truck-style SUV but have been scared off by poor mileage and high fuel prices.

2008 Chevrolet Tahoe Hybrid

•What is it? Gasoline-electric-power version of full-size, four-door, eight-passenger General Motors SUV, available with rear-wheel or four-wheel drive. GM also will sell a hybrid version of the similar GMC Yukon SUV.

•How soon? On sale fourth quarter.

•How much? GM won't say yet, but promises that the hybrids — which will be well-equipped, premium models — won't be the most expensive in the $35,000-$55,000 Tahoe/Yukon lineups.

•How many? GM is building 10,000 this year and says it's geared to supply as many as buyers want.

•What's the powertrain? 6-liter, aluminum alloy V-8 rated 332 horsepower at 5,100 rpm, 367 pounds-feet of torque at 4,100 rpm, coupled to a hybrid drive system housed in the transmission case and consisting of two electric motors and a planetary gear transmission, with manual-shift mode.

One motor helps drive the vehicle. The other helps synchronize the transmission with the power flow from the gasoline engine and electric drive motor.

A 300-volt battery pack under the second-row seats powers the electric drive motor and is recharged by regenerative (regen) braking, a process that sends power to the battery pack when the vehicle slows or is braked. The gasoline engine powers the recharging system when regen braking is insufficient.

•What else? Size, features are similar to gasoline-power Tahoe and Yukon models, which are detailed at www.gmbuypower.com.

Key exception: Hybrids tow less. Rear-drive hybrid is rated to tow 6,000 pounds, vs. 7,500 pounds for gasoline version. Four-wheel-drive hybrid is rated to tow 6,200 pounds vs. 8,200 for gasoline.

•How thirsty? GM forecasts 19 or 20 miles per gallon in town, 20 to 22 mpg highway and 20-21 in combined city-highway driving. Those are increases vs. gasoline-only models of about 40% in town, 5% on the highway, 25% combined. Official government ratings not yet available.

Four-wheel-drive, preproduction Tahoe test vehicle registered 18.2 mpg in about 70 miles of combined driving, including about 5 miles of light towing.

Gasoline model tested January 2006 registered 12.7 mpg in town, 17.3 mpg on the highway.

•Overall:Nicer than gasoline models and less odious to fuel.