Diesel Vs. Hybrid: We Debate Two Technologies

By Dan Edmunds, Director of Vehicle Testing Email | Blog
Date posted: 10-19-2008

Sure, you can get sticker shock at the gas station, but you don't have to give up your sport-utility vehicle. You don't even have to give up the carlike luxury trimmings you've come to expect as part of the SUV experience.

That's what the 2009 Mercedes-Benz ML320 Bluetec and 2008 Lexus RX 400h are meant for.

Or maybe you're looking to make a political statement. Those on the right-hand side of the aisle might want to use less imported oil, while left-leaning types just want to use less oil, period. And what better way to represent your social consciousness than with the SUV you drive?

In fact, this comparison is all about the debate. Once you commit to a greener way of life with a sport-utility, which is the better choice, diesel or hybrid? 2009 Mercedes-Benz ML320 Bluetec or 2008 Lexus RX 400h?

Point, Counterpoint
Representing the left is the progressive 2008 Lexus RX 400h — that's "h" as in hybrid. This RX features a beefier version of the Hybrid Synergy Drive system found in that darling of the eco-warrior set, the Toyota Prius. Depending on how hard you leg it, the RX 400h's gasoline-fueled 3.3-liter V6 plus its assemblage of electric motors and batteries can either put 268 horsepower to the pavement or return 25 mpg combined — 6 mpg more than a non-hybrid RX 350.

On the right is the more traditional and conservative 2009 Mercedes-Benz ML320 Bluetec, powered by a 3.0-liter V6 turbodiesel that pumps out 398 pound-feet of torque. The inherent efficiency of the diesel cycle means the ML320 Bluetec is rated at 21 mpg on the EPA's combined cycle, some 24 percent more than the otherwise identical gasoline-powered Mercedes ML350.

Ah, but this is no old-school soot-belching diesel. The Bluetec moniker identifies this ML320 as a diesel that meets the same tailpipe emissions standards required of gasoline engines sold in California — the toughest such diesel regulations in the world.

The Bluetec system combats oxides of nitrogen (NOx), the nasty byproduct of the diesel combustion cycle, by employing two catalytic converters. AdBlue, an aqueous urea solution stored aboard the Benz in a separate tank, is injected into a downstream exhaust catalyst where it reacts with the NOx in a process known as Selective Catalytic Reduction to form harmless nitrogen and water vapor.

Price and Prejudice
The price of our 2008 Lexus RX 400h, an all-wheel-drive model, starts at $44,305. When you pay $4,130 more, you get an integrated system that includes navigation, Mark Levinson premium audio, Bluetooth and a rearview camera. The interface is supremely easy to use and understand, but frankly we're baffled at the absence of even the most basic auxiliary jack to connect an iPod or MP3 player. Nostalgia fans may appreciate the included cassette deck, but we're not amused.

The RX's interior materials are very good, but the overall environment doesn't express luxury values quite like the Benz. The leather seats that come with the $1,600 premium package are soft enough, but the $380 trim in black bird's eye maple looks somehow inorganic.

Add $665 more for heated front seats and rain-sensing wipers and another $160 for the towing prep package (cooling system upgrades, not a hitch) and you arrive at this RX 400h's $50,885 as-tested sticker price.

The 2009 Mercedes-Benz ML320 diesel's price tag starts at $48,125 and quickly heads north. You'll pay $6,600 for Premium Package #2, consisting of navigation, Harmon Kardon premium audio that includes iPod and memory card inputs, Bluetooth, a rearview camera, proximity sensors and a host of power luxuries. We're not overly fond of single-point control strategies like Mercedes' COMAND system, but here it's nearly intolerable. The well-weighted control wheel found in our 2007 Mercedes-Benz C300 Sport long-term test car is absent, replaced here by a circular array of five flat buttons positioned on the passenger side of the center stack.

But there is no denying the high cush-factor that exudes from all the surfaces of the interior. Our test ML has $1,995 of gorgeous leather seats and exquisite wood interior trim, and every detail suggests a high level of craftsmanship. The $1,390 heating package includes heated front and rear seats and even a heated steering-wheel rim. A third zone of automatic climate control for the rear seat passengers costs $770. And then the whole thing is lathered in Alpine Rain metallic paint, a $720 option.

Of course, then you add another $1,600 for height-adjustable air suspension with adaptive damping. So the final damage on this luxuriously equipped utility vehicle is $61,200.

On the Move
We've driven Lexus RX hybrids before, and this one is much as we remember. Instantly accessible torque from the electric motor provides a good shove when you leave the line at a stoplight, as our 7.2-second acceleration to 60 mph (6.8 seconds with 1 foot of rollout like on a drag strip) can attest. In town, the computer-regulated transition from electric to gasoline motivation is seamless, and power flows smoothly through the shiftless planetary continuously variable transmission (CVT).

The braking performance of this Lexus, on the other hand, is decidedly average, as it takes a ho-hum 133 feet to come to a halt from 60 mph and the body dives noticeably. Speaking of hum, you'll hear a melodious hint of it from the regenerative braking system as it feeds power back to the batteries, especially when you ease to an unhurried stop. Think of it as the sound of fuel savings and maybe you won't be bugged by the annoying transition from regen braking to full mechanical retardation while you're in stop-and-go traffic.

The electric power steering is accurate enough, but the effort doesn't build much as the tires load up with cornering force, lending a somewhat numb feel. These fuel-friendly low-rolling-resistance tires also fail to generate much grip — just 0.68 lateral g on our skid pad — so the RX 400h's stability control system fires early in spirited driving.

On Your Right
The personality of the Mercedes-Benz ML320 Bluetec is about 180 degrees opposite to that of the Lexus. There is a slight hesitation when you jab the throttle pedal, but shortly thereafter the turbo boost builds, the torque comes on line and the big Merc accelerates smoothly through its seven-speed transmission. Our ML weighs 5,129 pounds, some 610 pounds more than the RX. As a result, its somewhat pedestrian acceleration to 60 mph in 8.5 seconds (8.2 seconds with 1 foot of rollout, like on a drag strip) is expected.

This extra mass doesn't seem to hamper stopping distance, as the Benz needs only 121 feet to halt from 60 mph. Wider 255/50R19 tires — more than an inch broader than the Lexus — doubtlessly play a role here. Bigger shoes also improve grip, and the heavier Merc posts 0.76 g on the skid pad and negotiates our slalom at 62.6 mph (a full 3 mph faster than the Lexus). There's more steering feedback, too, but the softly sprung air suspension lets the body roll too eagerly as you turn into a corner.

And that's not all. The use of air suspension also gives the ML a queasy ride, as it rocks a bit too easily from side to side over uneven pavement (the wife of one of us even vetoed a planned trip into the mountains after getting nauseous during a simple freeway cruise). Flicking the adaptive damping to Sport mode doesn't entirely quell the underlying floaty nature of the suspension; instead, it merely layers the occasional harsh jolt over the top.

We never thought we'd say this, but the smooth-riding Lexus feels taut and well-damped compared to the Benz. We recommend that ML shoppers save $1,600 and give the standard coil-spring suspension some serious consideration.

Utility Is the Middle Name
Both these SUVs lack the option of a third row, a state of affairs that allows both to optimize seat room for rear occupants. The Benz scores higher here, because its longer wheelbase (114.7 inches vs. 106.9) gives it a 3.6-inch advantage in rear-seat legroom. The Lexus has plenty, but the Mercedes seems like a presidential limo back there.

You'd think that the Benz would enjoy a cargo space advantage, too, but that doesn't appear to be the case. Lexus boasts that 84.7 cubic feet is available with the seats down in the RX, while Benz only claims 72.4 cubic feet of maximum cargo space. We suspect the Benz number assumes a conservative loading strategy that maintains sight lines out the rear window.

Both candidates offer all-wheel drive, but their systems differ dramatically. The RX 400h only has on-again, off-again electric motor power that goes to the rear axle, as the gas engine and CVT supply power exclusively to the front wheels. This means that the Lexus isn't suitable for off-road duty; its AWD system is best seen as a traction aid on snow-covered roads. Meanwhile, the ML has Benz's 4Matic always-on all-wheel-drive system with three differentials, electronically activated (through the ABS system) limited-slip action, as well as hill descent control. The ML doesn't have a low-range transfer case, so you can't take it to the Rubicon Trail, but you can take it off-road.

Towing is a huge area of difference between the hybrid and the diesel. Even with the towing prep pack, the RX 400h is rated for only 3,500 pounds of towing capacity. That's good for a couple of motorcycles or watercraft, but camping trailers and car haulers are out of its league. The ML320 Bluetec, on the other hand, is rated for 7,200 pounds of towing capacity — some serious capacity. Plus, a diesel usually preserves much more of its fuel economy while towing than a gasoline engine, too.

The Catch
But there's a problem with the Benz: It's got no spare tire. Why? The all-important AdBlue fluid tank has to go somewhere, and the only available spot apparently is the space under the cargo floor that usually accommodates a spare tire.

No spare means the ML320 Bluetec wears run-flat tires. Run-flats might sound like a great idea; nobody need worry about stopping on the side of a freeway or in a bad neighborhood (one without a Starbucks, presumably) if a tire goes flat. But finding an exact replacement run-flat tire within 100 miles of a tire failure isn't always easy in this massive country of ours. And they're not cheap to replace.

That's the car argument. For SUVs, the situation is even more critical. With the ML you could find yourself 30 miles down a dirt road or camping in a remote spot. Maybe you suffer your flat when visiting the north rim of the Grand Canyon with your 25-foot camping trailer in tow. The nearest city with a chance of having a replacement Bridgestone Dueller H/L 400 is Las Vegas, 266 miles away. Guess what? The run-flat range of an ML320 loaded to GVWR is only 20 miles. What do you do then?

We think that if you plan to use your SUV as an SUV, then you should have a spare tire — period.

The Results Are in
For most of our test, the 2009 Mercedes-Benz ML320 Bluetec led the Lexus in our comparison. Sure it's more expensive, but the level of equipment and the luxury feel of the cabin justifies the price. The diesel powertrain might not be quite as fuel-efficient as the RX in town, but it still delivers good urban fuel consumption numbers for a vehicle of its size, while its highway fuel economy is the same as the RX and its overall cruising range is superior. It even handles well (especially considering its 7,200-pound tow rating).

But the limited safety margin offered by the Benz's run-flat spare negates its adventure capability because we can't in good conscience recommend that you tow or go off-road without a spare. So what we're left with, then, is a comparison of two pavement-bound SUVs that are meant for the suburbs.

And on that basis, the 2008 Lexus RX 400h comes out ahead in this comparison. For thousands of dollars less, it's a well-equipped, smooth-riding luxury SUV with seamless power delivery, and it just happens to use less fuel than any garden-variety compact SUV. And isn't that the point of this debate anyway?

Second opinion:
Edmunds.com Associate Editor Josh Sadlier says:
The RX 400h's platform debuted way back in 2004, which is eons ago in car years. I still dig the exterior styling, but the cabin is really starting to show its age, from the tacky silver-painted plastic on the center stack to the featureless front seats with minimal power adjustments. Sure, the Benz costs an additional $10 grand as tested, but its sumptuous interior makes that premium seem almost reasonable.

But if I had to choose between them, I'd take the Lexus every day of the week.

The Benz's Bluetec engine is supposed to be the big news here — a clean-burning diesel that meets 2010 emissions standards while theoretically providing that classic diesel combination of fuel-efficiency and low-end kick. Yet the RX 400h's aging hybrid powertrain smokes the Benz's like a puff of old-school diesel exhaust. Sixty mph arrives more than a second sooner in the Lexus, and its responsive CVT delivers a heady rush of electrically assisted power on demand. The RX trounced the ML320 in observed fuel economy, too, and the premium gas it requires is about 30 cents cheaper than diesel as of this writing.

In other words: Diesel, schmiesel. And the Lexus steers better to boot. Make that much better — the RX's electric power steering system is slop-free and nicely weighted, while the Benz's gooey hydraulic setup feels about as precise as the helm of a commercial fishing vessel. These vehicles aren't built with enthusiasts in mind, of course, but if you enjoy driving, there's only one horse in this race.

Yes, the Lexus feels old. But it's both quicker and more fuel-efficient than the Benz, its steering is superior, it's considerably cheaper, and oh yeah — its concert-quality Mark Levinson stereo puts the Merc's Harman Kardon system to shame. I'll certainly stay tuned as the Bluetec lineup evolves, but for now, the hoary Lexus hybrid reigns supreme.