The explanation seemed plausible enough.

Despite having a French-sounding last name, two video cameras with night-vision capability and a car that belonged to someone else, what could a man standing on a deserted road in Upstate New York, just a few feet from the Canadian border, possibly be up to other than conducting a car review?

The first agent of the Customs and Border Patrol bought it, and gave me five minutes to finish the little TV thing I was in the middle of, as long as I promised to notify his office the next time I planned on tripping whatever sensors must have tipped them off.

Read more about the Mercedes-Benz E320 and other cars at the Fox Car Report section.

Unfortunately, his colleagues back at HQ didn't get the message, and after five more government-issue SUVs pulled up, he decided he’d need to fill out an incident report.

The 15 minutes I had to wait wouldn’t have been such a big deal if I wasn’t already six hours into a 12-hour experiment, my long day's journey into determining whether a Mercedes could go 700 miles on a single tank.

But I knew going into this that I had a long day ahead of me.

When Mercedes-Benz sent over one of its 2008 E320 BlueTec sedans, I was anxious to see how well the latest diesel-powered luxury car from Germany would stack up to the gas-burning competition. Along with a number of automakers, including Audi and Honda, Mercedes-Benz is looking at diesel to help the company meet the drastically increased fuel-efficiency standards heading our way.

• Click here for photos.

First, forget everything you know about diesel cars, because the situation is much different now than it was in the 1980s, the last time they made any significant impact on the American marketplace.

Government regulations today require diesels to meet the same emissions standards as their gas-powered counterparts. So there’s no more smoke, no soot and no eau du tractor stink as they go about their business.

Now you can go ahead and remember the good things about diesel, the outstanding fuel efficiency combined with the tire-spinning power of a muscle car, because they still apply.

Two great things that go great together, but usually don’t.

On paper, the E320 stacks up well against its closest gas-powered competitor, which happens to be Mercedes-Benz’s own E350. They look exactly the same, have identical interiors and each accelerates just as quickly as the other. At $53,025, the E320 does cost $1,000 more, but it’s actually cheaper to own in the long run.

With an EPA rating of 32-mpg highway/23-mpg city, the E320 gets 35 percent better mileage than the E350, at 24/17, and is the most fuel-efficient luxury car you can buy. Even factoring in the higher price for diesel fuel, an E320 owner should make up the premium she spent on it in less than three years. After that, it’s money in the bank.

Although I wanted to have a normal week with the E320, putting it through its paces like any other car, a number of things got in the way of my plan.

Seven hundred of them, to be exact.

E-class sedans have rather large gas tanks, over 21 gallons, with a little extra in reserve. Your car is probably closer to 15 gallons. Even an engineering school drop-out like me could calculate that, on a good day, with a long enough stretch of highway, the E320 should be able to travel 700 miles between fill-ups.

Some ideas are too good to pass up, others so bad you just can’t help yourself. So I plotted a course that would take me north from New York City and up Interstate 87 to the Canadian border, where I planned to cross over for a quick lunch before heading back to Manhattan in time for bed.

Watch the epic journey.

Sure, I could’ve taken seven 100-mile trips, but it wouldn’t have been the same. Besides, few cars ever live up to their EPA estimates in normal driving. To pull off this feat, I’d have to get on a highway, set the cruise control at the speed limit, and sit back and enjoy the ride.

For the most part, I did just that. In any of its forms, the E-class sedan is an outstanding luxury car, and the diesel version is no different.

Instead of leather, the seats in my test vehicle were upholstered in a synthetic material called MB-Tex, creating an animal-free interior for buyers who want to save the world and everything in it.

Of course, cows have emissions problems of their own, so if the atmosphere is your main concern, you may want to opt for the leather seats. MB-Tex is fine, though, and actually feels a little softer than the natural hides, if not quite as elegant.

The engine, a turbocharged 3.0-liter V6, is powerful, offering performance on par with most gas-powered motors in its class, but don't mistake the E320 for a sports sedan. The suspension is on the soft side, and the tires are high-mileage, comfort-oriented pieces that don’t have a ton of grip. Take a sharp turn above the recommended speed and you’ll think twice about trying that again.

With more torque on tap than you get in a Ferrari F430, the E320 is plenty quick from a stoplight, though, and passes slow-moving vehicles at highway speeds with no fuss. But if you see a BMW 5-series in your rear-view mirror on a twisty mountain two-lane, you’d be advised to move to the right.

No such roads are found on I-87, and by the time I reached Albany, the state capital of New York, 150 miles into the trip, I was averaging over 33.2 mpg traveling at a steady 65 mph, noting an interesting irony as I did.

Although the E320 meets EPA emissions regulations, those crazy kids in California have stiffer rules than the ones imposed by D.C., and no diesel cars currently meet those standards; however, for 2009 Mercedes is updating its engines so they can inject a substance called AdBlue — hence BlueTec — into the exhaust stream, that chemically cleans it up to meet even those strict regulations.

New York happens to be one of eight states that follow California’s rules, so you can’t buy the E320 there, making my trip a bit of an anomaly. If your governor is happy to take his orders from Washington rather than Sacramento, you’ll have no problem ordering one at your local Mercedes-Benz dealer.

After 300 miles the car’s computer indicated that it was getting 33.7 mpg, and Canada was in my sights. Sadly, I’d overlooked the clause in my loan agreement that prevented me from actually entering our northern neighbor with it, leading to my run-in with the Border Patrol. I settled for McDonald’s fries for lunch, instead of poutine, but this minor roadblock would cause major problems for me at the other end of the trip.

It was at this point, nearly seven hours in, that my relationship with the car had reached the point where the nitpicking set in, but there really wasn’t much to complain about. Like all E-series, there’s a beautifully finished sliding door covering the storage compartment in the center armrest, making it the last place in the car that you would want to rest your arm.

The door also covers the cup holders, which are positioned directly behind the gear selector. If you put anything taller than a cup of coffee in there it gets in your way when you need to shift.

Otherwise, the E320 is a fantastic long-haul companion. While there is a little of the familiar diesel clatter at idle, on the road the engine is as silent as any other. The only noise in the cabin comes from the wind and the road, and there’s not much of either of those. You can actually hear light gusts of wind hit the car, the normal rush of air that accompanies a car at a speed is that subdued.

As far as those seats are concerned, I have back trouble that usually kicks in after less than two hours behind the wheel of most cars. After 10 in the E320, I was feeling no pain. At that point, it was averaging 33.8 mpg, and that figure was up to 33.9 by the time I crossed the 600-mile mark, but the car was telling me there were only 97 miles left in the tank.

Cue the dramatic music.

At 650 miles, the range was down to 46 miles and I realized that since I hadn’t gone all the way into Canada, my route back to the office wasn’t going to add up to the full 700 miles. Needing to stay on the highway I took a detour — into New Jersey.

Cue the Bruce Springsteen.

As I made my way down I-95, the computer threw in the towel and, instead of showing me a mileage figure for the remaining range, it projected a picture of a car parked next to a gas pump.

At 680 miles I had gone far enough to make up the distance deficit, and I turned north to start back toward Manhattan, passing by a refinery as I did.

Rub it in, Big Oil, rub it in.

The sweat kicked in at the thought of breaking down on the New Jersey Turnpike at 10 p.m. in freezing temperatures — an experience I'd already lived through once in my life — but I’d taken the leap of faith, with no service stations left between me and 700 miles.

As 699 miles approached the E320 was averaging 34.6 mpg and I went through the 700-mile mark with a shout of victory, a sigh of relief and the somber realization that I still needed to find a gas station that sold diesel before I was in the clear. I wanted to slam on the gas…uh, accelerator pedal, and drive as quickly as I could to the next exit, but decided to inflict slow torture on myself by setting the speed at 55 mph to conserve fuel.

If there’s a downside to owning a diesel, this is it. Services stations that sell it can be few and far between. Even in an area heavily trafficked by oil-burning trucks, I found this out the hard way.

The odometer read 706 miles, the navigation system showed an icon for the gas pump I was heading for a mile farther than that. After 12 hours behind the wheel, I would be spending one more in a Toys-R-Us parking lot waiting for the roadside assistance to arrive.

At least I had a nice place to wait.

—————

2008 Mercedes-Benz E320 BlueTec

Base Price: $53,025

Engine: 3.0L Turbocharged V6 Diesel

Power: 210hp, 400 lb-ft torque

Drivetrain: Rear-wheel drive w/7-speed automatic transmission

MPG: 23city/32hwy

0-60mph: 6.6 sec

What do you think about the Mercedes-Benz E320 BlueTec?

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