The Good: As a full-size SUV that gets more than 21 mpg because of its diesel engine, the 2008 Mercedes-Benz GL320 CDI costs a lot less to operate than competing luxury SUVs. An adjustable suspension and off-road mode make it suitable for rough terrain. The Bad: The stereo doesn't display track information for MP3 CDs and the infotainment interface's looks are as bad as its performance. The Bottom Line: The cabin electronics and interface for the 2008 Mercedes-Benz GL320 CDI's infotainment system are old and have fallen far behind today's standards, but the diesel engine makes the car drive well and get incredible fuel economy for a full-size SUV. Photo gallery:2008 Mercedes-Benz GL320 CDIWith its diesel engine, the 2008 Mercedes-Benz GL320 CDI transports seven people and plenty of cargo while getting mileage worthy of a midsize sedan. The oversize SUV from Mercedes-Benz features all the interior luxury we would expect from the German carmaker, along with performance that favors economy over power. Unfortunately, it uses the same baseline cabin gadgets we saw in its gasoline-engined counterpart, the Mercedes-Benz GL450. This system covers a lot of the basics well enough, but the interface is poor and some features are antiquated. Test the tech: Wine country road trip A large SUV sporting the Mercedes-Benz tri-star belongs in California's wine country, where upscale types pretend to rough it as they visit wineries and buy cases of the latest pinot noir or sauvignon blanc. To test out the GL320, we took it on a trip to its natural environment, plotting a course 90 miles north from San Francisco to Lake Sonoma. With the navigation system, we searched the POI database using Lake Sonoma as our search term. It found the recreation area and calculated our route. The big SUV was unwieldy in the city, but once we got on the freeway it seemed more in its element. Keeping up with traffic and passing slower cars proved easy, as the seven-gear automatic kicked down to lower gears when we floored it, the tach rising up to the 4,000rpm line. Yes, it's a diesel, so redline is at 4,500rpm. Unlike diesels of the past, the GL320 didn't produce excessive noise or a particularly bad odor. We had an easy cruise in this luxurious box up Highway 101 until the navigation system told us to take the Canyon Road exit, which turned out to be a nice country road through vineyards. A right on Dry Creek Road and we were in serious wine country, with vineyards and wineries at every stop. But we continued on our way to the Lake Sonoma Recreation Area, stopping to enjoy the views and stretch our legs. The GL320 enjoys the view at Lake Sonoma.When we got back in the GL320, we explored a little farther up a fantastic winding mountain road. But we quickly determined we were in the wrong vehicle for these types of exploits, as the GL320 wasn't making us comfortable in the corners. So we turned around before some sports car out for a spin caught up with us. We followed Dry Creek Road by numerous wineries to Healdsburg, where we reconnected with the highway. The car had been a reasonable cruiser off the beaten path, and we're sure its off-road gear, including an adjustable air suspension, could have handled some rougher terrain, but it wasn't really made for the twisties. We completed our trip back to San Francisco on Highway 101, doing speeds of around 70 mph. Then back in the city we traversed the tight streets and made it back to headquarters. Assessing our trip, we drove 189 miles and got 23.8 mpg. We didn't need to fill up the tank, as we barely got the fuel gauge needle down by a quarter, but checking the current diesel prices around San Francisco, we noted it would have cost us $3.65 a gallon. Using 7.94 gallons of diesel for our trip, our fuel cost would have been $28.98, a paltry amount compared with what you would pay in its gas-engined counterpart, which gets an average of 15 mpg. In the cabin Inside the GL320, you get the usual refinement offered by Mercedes-Benz, with good-looking gauges and switchgear. But the ugly plastic buttons surrounding the navigation screen work against the effect, and we just don't care for the particular shade of tan used through most of the interior. Unlike Mercedes-Benz sedans, which put the seat controls in an easily accessible spot on the door, the GL320 has them on the side of the seat, just like most other carmakers, taking away some of what makes a Mercedes-Benz special. The stereo displays file names but no song information from MP3 CDs.The GL320 uses the same infotainment interface, with its washed-out blue color scheme and fiddly knob\/joystick, as we've seen on previous Benzes. We can only hope that soon the system will be updated with what we saw in the Mercedes-Benz C300. This system controls the navigation and audio functions, both of which are fine, but offer nothing special. The navigation system has perfectly good route guidance and a limited database of POIs. But it lacks advanced features such as text-to-speech, and the map resolution isn't good. We do like that you can enter multiple addresses and get it to compute the best route. As for the Harmon-Kardon Logic7 stereo, a premium upgrade, it sounds very good, but doesn't really have enough speakers to fill the cavernous interior. On our car, we had a single-disc slot behind the LCD and a six-disc changer in the glovebox. Both played MP3 CDs, but we were disappointed to see that only file names were displayed on the LCD, rather than full ID3 track information. We also had Sirius satellite radio. The back-up camera's image is very clear.As for hands-free calling, the GL320 is still stuck with a dedicated cell phone cradle system, as we've seen on past Mercedes-Benzes. More recently updated models are going to Bluetooth wireless connections. Our GL320 came with a rear-seat DVD option. The screens, set behind the head rests on the front seats, are well-placed and look good. But the DVD player itself is set under the right rear passenger seat, making it impossible to see. You have to insert and eject DVDs, or plug other audio\/visual gear into its RCA jacks, by feel. Parking is helped by one of the clearest rearview cameras we've seen, although it lacks any overlay to show how close you are to an obstruction or where your wheel turn will take you. And being a big luxury SUV, it has niceties such as a powered lift gate and buttons to make the third-row seats fold flat or open up. Under the hood We had previously reviewed the 2007 Mercedes-Benz GL450, which uses a 4.6-liter V-8 instead of our GL320's turbocharged 3-liter diesel V-6. The GL450 felt fast, jumping quickly off the line. The diesel-powered GL320 feels a little more sluggish, as if all of its components need to make a collective decision to accelerate when you hit the throttle. But after this initial hesitation, it builds speed nicely. Most people wouldn't even realize it has a diesel engine. This engine makes 215 horsepower and 398 ft-lbs of torque, that peculiar diesel mix that works very well in a vehicle that weighs in at 5,313 pounds. This 3-liter diesel engine has no problem moving the big GL320.We covered our mileage on a trip (above) that favored highway driving. Overall we saw an average of 21.2 mpg, landing right in between the EPA's estimated 18 mpg city and 24 mpg highway. Emissions ratings weren't published at the time of this review. The GL320 uses Mercedes-Benz's seven-speed automatic transmission, with a manual selection mode activated by buttons on back of the steering wheel spokes. In this application, the transmission lets the engine get up to around 4,500rpm before shifting under heavy acceleration. During our driving, we never felt this transmission make aggressive downshifts, but the GL320 isn't the kind of vehicle that could make use of it. In corners, we felt body roll that suggested we shouldn't push it very fast. We saw an average of 21.2 mpg for the GL320, an impressive feat for such a big vehicle.No, the GL320 is a big SUV, with Mercedes-Benz's 4Matic all-wheel-drive system and a couple of tricks to help it out in off-road situations. Although ours didn't come with the off-road package, its air suspension lets you raise it up to its highest clearance, at 10.9 inches. Another button lets you put it in general off-road mode, designed to maximize traction. It also has hill descent control, and it even lets you set the speed for this function. And the safety equipment is complete, as we would expect from Mercedes-Benz, with traction and stability control. In sum The 2008 Mercedes-Benz GL320 CDI comes in at a base price of $53,000. Options on our test car included the Premium II Package for a whopping $8,600, the $2,100 full leather package, and another $2,670 for the rear-seat DVD system. Along with $700 for the paint and the $775 destination charge, the total came out to $67,855. The Premium II Package includes most of the cabin gadgets, such as Sirius satellite radio and navigation. The GL320 is a pricey package, but if you're looking for a full-size luxury SUV, its diesel engine efficiency represents an incredibly compelling argument. Compared to its gasoline counterpart, the GL450, or Land Rover's Range Rover, the GL320 can transport the same number of people and as much cargo. Both gasoline SUVs have more horsepower, but the high torque of the GL320 gives it an edge in this high weight category. If you insist on gasoline, you can get the same level of luxury, mileage, and a little bling with next year's Cadillac Escalade Hybrid. Our only reservations about the GL320 are the cabin electronics, which should improve substantially with the next model update.