Comparison Test: 2008 Chevrolet Tahoe Hybrid vs. 2008 Mercedes-Benz GL320 CDI

No one's talking about it, but the 2008 Chevrolet Tahoe Hybrid and 2008 Mercedes-Benz GL320 CDI are the two most fuel-efficient full-size SUVs on the market. We guess that makes Paris Hilton, proud owner of a 2008 GMC Yukon Hybrid, the hybrid Tahoe's twin, a forward-thinking early adopter at the vanguard of the green car movement.

Truthfully, calling either of these SUVs "green" might border on an insult to the color, not to mention the movement. These are still large, comfortable vehicles with more torque than the average commuter needs and more seats than most families will use, perfect for vacations but decadent in daily life.

But these vehicles are more reasonable when you consider that the four-wheel-drive 2008 Chevrolet Tahoe Hybrid earns an EPA rating of 20 mpg city/20 mpg highway, and the all-wheel-drive 2008 Mercedes-Benz GL320 CDI rates 18 mpg city/24 mpg highway. This works out to a combined rating of 20 mpg for both SUVs — a big achievement for a full-size sport-utility.

When it comes to large sport-utilities, think of this as a practicality test for hybrid and diesel. In other words, if you want to drive something big without feeling bad, this is what it's all about.

Why a Chevy Can Compete With a Benz
Brand prejudice says we're making a big mistake comparing a 2008 Chevrolet Tahoe Hybrid to a 2008 Mercedes-Benz GL320 CDI. Maybe you're saying this, too. But with a base price of $53,295 for the hybrid Tahoe and $53,775 for the diesel GL, the cost difference is all of $480.

Of course, our test Benz vaporizes this illusion of parity because we've had to use a test vehicle that's equipped with more than $15,000 in options, including the $8,600 Premium II Package, which loads it with everything from a navigation system to adaptive bi-xenon headlights. An expensive dual-screen entertainment system ($2,670) and a nice (but not that nice) set of 19-inch wheels ($1,520) chip away at any remaining value until the GL320 CDI tops out at $69,375.

After reviewing the equipment lists of the Tahoe and GL, though, we've decided the game's still on. Put aside the GL320's leather-upholstered dash and power liftgate, and these two SUVs are comparably equipped. Our Tahoe Hybrid has a navigation system, rearview camera, leather upholstery and simulated maple trim as standard. With the optional single-screen entertainment system ($1,295) and sunroof ($995), it costs $55,585 — or $53,385 after you file your 2008 tax return. With the tax credit included, it's about $5,500 more expensive than a comparably equipped regular Tahoe.

And you could order a 2008 Mercedes GL320 CDI for Tahoe Hybrid money without it feeling like basic transportation. You'd just sit on leatherette seats, learn how to read paper maps and listen to All Things Considered through eight speakers instead of 11.

Tahoe Hybrid: One V8, Two Electric Motors and a Whole Lot of Gear Ratios
Money isn't really what separates the Chevrolet Tahoe Hybrid and the Mercedes-Benz GL320 CDI anyway. Rather, these full-size SUVs represent two completely different approaches to cutting fuel consumption.

In the case of Chevy's Tahoe Hybrid, it's a "more is more" argument. You start with a 6.0-liter gasoline-fueled V8. Like the 6.2-liter V8 in the Cadillac Escalade and Yukon Denali, this one uses an aluminum block and variable valve timing, and has a fairly tall 10.8:1 compression ratio. It also has GM's Active Fuel Management, which allows it to run on four cylinders while cruising at light throttle loads.

Like most engines involved in hybrid applications, this 6.0-liter V8 runs on the Atkinson cycle, which keeps the intake valves open during part of the compression stroke. This reduces compression volume, improving fuel economy at the expense of power and torque. This also explains why this V8 makes only 332 horsepower at 5,100 rpm and 367 pound-feet of torque at 4,100 rpm — somewhat tame figures for a GM V8 of this displacement.

To help make up for this, the two-mode hybrid gives you a pair of 60-kilowatt electric motors living in the complicated assembly that makes up the transmission. They draw juice from a 300-volt nickel-metal-hydride battery pack packaged under the second-row bench seat.

Electric motors can deliver instant torque response off idle, provided there's an efficient way of getting it to the wheels. And that's where the Tahoe Hybrid's two-mode transmission design comes in. It has three planetary gearsets, which provide the equivalent of two variable forward "gears" (along with a third for reverse). They're used when this SUV's running on electric power alone or (more commonly) on a combination of electricity and gas. As a result, it sometimes feels like you have a continuously variable transmission (CVT) in your Tahoe Hybrid.

When the Tahoe is being propelled by its V8 engine alone — which is most of the time — the transmission performs like a conventional automatic. This element of the transmission has four forward ratios and they're what you notice once you're up to cruising speed. This unusual combination of planetary gearsets and conventional gearing makes it possible to accommodate a true 4WD system with low-range gearing, something no other production hybrid has.

Mercedes-Benz GL320 CDI: The Simple Life
It's funny to think that a Mercedes could be less complex than a Chevrolet, but all we're talking about in the GL320 is a turbocharged, intercooled 3.0-liter V6. Of course, it's got ceramic piezo injectors instead of spark plugs, and it has a 16.5:1 compression ratio that would be unthinkable on a gas engine. That's right, we're talking diesel.

An oxidation catalyst and a particulate filter cut down on the soot emissions that make people think diesels are dirty, while pilot injection (a preheating of the combustion chambers using small amounts of fuel before the primary injection) and exhaust gas recirculation minimize the nitrogen oxides (NOx) coming out the back. Still, these measures are only good enough to certify the Benz for sale in 42 U.S. states. It will take the AdBlue urea tank in the otherwise identical 2009 Mercedes-Benz GL320 Bluetec to clean it up for all 50 states.

Like every other member of the Mercedes GL-Class family, the GL320 CDI uses a seven-speed automatic transmission, which drives all four wheels through full-time all-wheel drive. With only a single power source, this seven-speed usually feels smoother and more responsive than the Tahoe's transmission.

Can They Hit the EPA Ratings?
Once we're on the road, we forgive the odd sounds and sensations from the GM hybrid drivetrain when the Chevrolet Tahoe Hybrid returns 20.1 mpg during our 120-mile city loop — a stop-and-go diet of surface streets in Southern California through Los Angeles and Ventura counties.

Although most strong hybrids do their best work in the city, this Tahoe Hybrid proves more fuel-efficient on the highway. We drive at a 70-mph pace on a 130-mile loop from the coastal plain in Santa Monica to the 3,500-foot pass at the crest of the Tehachapi mountains and back, and the Tahoe averages 21.9 mpg.

This makes sense when you understand the thinking behind GM's hybrid system. It's not so much about electric-only operation, as in the Toyota Prius. Rather the electric motors allow the V8 to stay in its Active Fuel Management V4 mode for longer periods than it might in a regular Chevy Tahoe, and the transmission picks ratios accordingly.

Since we're avoiding rapid acceleration and using cruise control whenever possible on the highway loop, there are fewer changes in throttle position than in city driving — which makes life easier for the transmission.

Our 2008 Mercedes-Benz GL320 CDI gets 26 mpg on the highway loop, which is a superb figure for a large SUV but not altogether surprising since the diesel carries a 24-mpg EPA highway estimate.

Harder to explain is the Benz's 22.1-mpg average on the city loop, a clear advantage over the Tahoe.

Partly, this might be due to one of the little discussed advantages of the diesel engine. That is, it doesn't require a conventional throttle. Basically, a diesel draws in the same amount of air on every stroke. So during city driving, the Benz turbodiesel V6 gets lots of air even though only small amounts of fuel are being injected. In contrast, a gasoline engine might as well be sucking air through a straw under light acceleration, and this creates more friction, resulting in pumping losses and poorer fuel economy.

However, we suspect lower curb weight is the primary reason for the GL320's mileage advantage. At 5,500 pounds, it's hardly svelte, but the Tahoe Hybrid weighs a monumentally chubby 5,900 pounds.

Equals in a Contest of Speed
In spite of its bulk, the Chevrolet Tahoe Hybrid forces itself through space with a haste that's comparable to the Mercedes GL. Both SUVs have some off-the-line snap, though neither is particularly exciting when you floor the gas pedal.

At our test track, the Tahoe Hybrid and GL320 CDI hit 60 mph in 8.8 seconds and stay right with each other through the quarter-mile at 16.4 seconds. The Chevy is moving at 86.8 mph at this point, 0.3 mph faster than the Benz.

For perspective, these numbers are on par with a normal Tahoe, which weighs 400 pounds less. Our long-term '07 Tahoe ran to 60 mph in 8.5 seconds and completed the quarter-mile in 16.4 seconds at 85.3 mph. Mercedes doesn't offer a gasoline V6 in the GL-Class line, and as you'd expect, the V8-equipped GL450 is significantly quicker than the diesel, reaching 60 mph in 6.7 seconds and completing the quarter-mile in 15.1 seconds at 91.6 mph.

Although acceleration is a draw, the GL320 can tow more. With its optional $510 tow package, it has a 7,500-pound tow rating compared to a 6,000-pound limit for the Tahoe.

The Agile Benz Pulls Ahead
Start going around corners and the Tahoe Hybrid's 3 tons are harder to hide. GM's hybrid SUV behaves predictably and its steering is surprisingly precise, but it turns in very slowly and braking effort is considerable, even though it stops from 60 mph in a respectable 134 feet, the same distance as our long-term '07 Tahoe.

Hop into the diesel GL and you realize the benefit of not only fewer pounds, but also of unibody construction and four-wheel independent suspension. In fact, the Mercedes-Benz GL320 CDI handles phenomenally well for a full-size SUV. Its 60.0-mph speed through our test slalom is the best number we've ever gotten out of a full-size SUV. Compare this to the Tahoe Hybrid's 57.5 mph.

Although the skid-pad numbers are close for these sumo-size SUVs — 0.73g for the Mercedes and 0.69g for the Chevy — the margin would be even more in the GL's favor if you could disable the Benz's stability control.

As on the Tahoe, the GL's brake pedal initially feels soft. As you get into its travel, though, the braking action progressively gets stronger. More important, the GL's braking distances are shorter, as the Mercedes stops from 60 mph in 123 feet.

The Germans Know You Better Than You Think
Although the 2008 Chevrolet Tahoe Hybrid has more hip- and shoulder room in the first two seating rows (as well as more storage space), the 2008 Mercedes-Benz GL320 CDI is the better packaged vehicle if you're going to use all three rows. Its independent rear suspension allows a lower floor and a more comfortable, higher-mounted third-row bench. Plus, that bench can fold away into the floor, improving cargo capacity.

The ergonomics of the driving position are acceptable in both SUVs, but the Tahoe has an edge here, as its touchscreen navigation interface is easier to use than the GL320's setup, which is the previous generation of the Mercedes-Benz controller-operated COMAND setup.

Diesel Wins This Round
Although the 2008 Chevrolet Tahoe Hybrid is more versatile than earlier hybrid SUVs like the Ford Escape and Lexus RX 400h and feels more normal to drive (vastly so, in fact), we prefer the 2008 Mercedes-Benz GL320 CDI, which feels like even less of a compromise.

The diesel GL also handles and stops better, while offering superior seat comfort and utility. And with its combined fuel-economy average of 23.9 mpg offering a substantial margin over the Tahoe's 20.9 mpg, the Benz easily wins our fuel-economy test, too.

Of course a comparison of fuel prices equalizes the game a little. During this test, we pay $3.579 per gallon for the Chevy's 87-octane gas, while the Benz's diesel costs us $4.179 per gallon. But because of the GL320's lower fuel consumption, we actually spent only a dollar more overall to fill its tank at the end of our test.

Availability is likely to be the bigger issue with both of these vehicles, however.

Until the Bluetec versions arrive this fall, getting your hands on a GL320, or any other diesel Mercedes SUV, is impossible in big states like California and New York. Curiously, buying a 2008 Chevrolet Tahoe Hybrid might not be much easier, unless you're Paris Hilton. GM says it has capacity to build 8,000-12,000 Tahoe/Yukon Hybrids this year, but we hear only 655 were sold in the first quarter of 2008. For now, you're more likely to see these SUVs in Mother Jones ads than on the road.

Nevertheless, this test offers reason for optimism. If these heavyweights can break 20 mpg, the same is true for all the seven-passenger crossovers out there. So let's keep the hybrid-versus-diesel battle going. It's our Obama vs. Clinton, and the longer it lasts, the better off we'll be.

The manufacturers provided Edmunds these vehicles for the purposes of evaluation.