comparison

Some automakers have had long-standing plans to offer diesel engines in this country. European automakers have stuck with those strategies despite the volatile price disparity between diesel and regular fuel. Japanese and domestic automakers, including Honda, Nissan and GM, have shelved their diesel plans for now.

Impressed by diesels we’ve previously tested from Mercedes-Benz and Volkswagen, we have added three more to our test fleet to see how they compare to their conventional counterparts. Our latest diesel models include the Mercedes-Benz GL320 Blue Tec and a pair of BMWs--the X5 and the 335d.

Mercedes-Benz GL320 Blue Tec
Mercedes-Benz-GL320-BlueTECWe were impressed with the GL450 when we tested it in 2006--a roomy, comfortable, yet responsive handling vehicle. We were also impressed with the 210-hp, 3.0-liter V6 diesel engine when we tested it in the E320 Blue Tec, which combined strong performance and refinement, yet returned 29 mpg overall. It was also free of traditional diesel smell, smoke, and sluggishness. Hence, we wanted to see how this engine performed in the GL.

Even though the GL is very heavy (5,655 lbs.), this engine remains smooth and muted, but initial off-the-line acceleration is sluggish. Once underway, it pulls strongly and manages to hit 60 mph in less than 10 seconds--a far cry from the 7.4 seconds we recorded for the GL450. Observed fuel economy, so far, is averaging 20 mpg overall. We got 15 mpg for our V8-powered GL450. If you’re starting to crunch the numbers, you should know that the GL320 is actually $1,000 cheaper than the GL450.

The other major difference between the two is that the GL320 comes with 20-inch run-flat tires, which brings more impact-harshness to the ride quality. Plus, the steering feedback is compromised. The GL benefits from a $1,550 federal tax credit for the first 60,000 purchasers, but you won’t qualify if you hit the AMT tax.

BMW X5 3.0 diesel
2009-BMW-X5In contrast to the Mercedes example above, the X5 diesel adds an eye-opening $3,700 to the price of the 265-hp, regular-gas 3.0 model. So far, our diesel X5 is getting 23 mpg on average, compared with 17 mpg overall for the gas version. The X5’s diesel is not quite as refined as the Mercedes but feels punchier. In fact, it beats the regular X5 in 0-60 mph acceleration, but it’s less quick initially from a standstill. Other than that, the diesel X5 retains the choppy ride, excellent steering and agile handling (as well as confusing controls) of the standard X5.

Diesel and gasoline are about equal in price these days, so it’s a matter of how far you drive when it comes to payback time for the diesel version’s higher price. There is also added maintenance due to the replacement of the Urea solution (also required with the Mercedes), which serves in emission-cleaning arsenal. BMW replaces it at service intervals and will warn you 1,000 miles prior. This diesel qualifies for a $1,550 federal tax credit. With that, and at current gas prices, and with an annual mileage of 12,000 miles it would take more than four years to break even.

BMW 335d
BMW_335d On the face of it, there is a $2,275 premium for a 335d over the rear-wheel drive 335i, once adjusted for the standard automatic transmission in the diesel. However, assuming your attitude is one of saving money and fuel, you’d probably look at the more popular 328i. In that case, the diesel commands a whopping $8,975 price premium. So far we’ve been getting 33 mpg with the diesel according to the car’s trip computer. Our last 328i attained 23 mpg overall.

Beyond the questionable economics, the diesel 3 retains its excellent steering and body control and one might argue is even more of a hoot to drive. We haven’t completed our acceleration tests yet, but the torque of the diesel delivers an addicting mid-range rush that, if you’re not careful, will send the car into three-digit speeds all too easily. The 335d benefits from a $900 federal tax credit. With that taken into account it would take about 20 years to break even compared to a 328i at an annual mileage of 12,000.

+ 2009 Mercedes-Benz GL320 BlueTEC Road Test
+ 2009 BMW 335d Fuel-Efficient Sedan Review
+ 2009 New York Auto Show: 2010 BMW X5 M

Bottom line
In general, diesel engines cut fuel consumption by about 30 percent and as a byproduct reduces C02 emissions. And contemporary common-rail diesel combined with devices that abolish smoke and smell make the engines a lot more livable. That said, in terms of particles causing soot, diesels are still not as clean as modern gas engines.

Opting for a diesel in today’s environment may not save you money. It is a function of the price differential to the conventional version, the price of diesel fuel and your annual mileage. Other than economics, any diesel can be fueled with an up to 20 percent bio-diesel blend, which further cuts petroleum consumption. If diesel prices remain similar to gasoline, as they are today, there would be true appeal.

Gallery: 2009 Mercedes-Benz GL320 BlueTEC



2010 BMW X5 M Gallery



Gallery: 2009 BMW 335d