MERAN, Italy – You can make a case for putting BMW AG’s powerful and efficient 3.0L turbodiesel in just about anything on wheels, but nowhere does it make more sense than in the BMW X5 xDrive cross/utility vehicle.

Sharpening your pencil and trying to use fuel-cost savings alone to justify the $4,000 premium for the $52,025 diesel version of the X5 xDrive 35d vs. the $47,925 I-6 gasoline-powered xDrive 30i is not a compelling case to penny pinchers. At least not when diesel still costs more than even the premium gasoline BMWs require.

A $1,550 income-tax credit sweetens the deal. But when you are spending $50,000 on a luxury CUV, pinching pennies is not likely your top concern. Value, performance and being greener than the neighbors are more plausible priorities, and here the diesel shines with 25% better fuel efficiency and a whopping 425 lb.-ft (576 Nm) of torque at 1,750 rpm.

Whether navigating winding mountain roads, barreling down the expressway at high speeds or simply chugging around town, the diesel option stands out as the most rational choice when the alternatives are an overmatched I-6 gasoline engine or a potent-but-thirsty 4.8L V-8.

Final fuel-economy ratings from the U.S. Environmental Protection Agency are not expected to be released until after the xDrive 35d debuts in production trim at the Los Angeles auto show, this week, but city/highway estimates are 19/26 mpg (12.4-9.1 L/100 km). Compared with the similar engines, that is excellent efficiency.

It also is superior to the diesel-powered Mercedes ML320 CDI’s 18/24 mpg (13-10 L/100 km) and slightly behind the $43,000 all-wheel-drive Lexus 400h hybrid-electric vehicle’s 26/24 mpg (15-16 L/100 km). However, Mercedes only charges about a $1,500 premium for a Bluetec diesel on its CUVs.

Like all BMWs, the X5 delivers precise handling and braking. On pavement, it can run circles around most CUV and SUV competitors. Even off-road, where virtually no BMW owners are likely to tread, it is a surprisingly deft performer.

But all this capability and ruggedness comes at a price: Its extra-strong body adds weight and hurts fuel economy and acceleration.

The 6-cyl. gasoline X5 tips the scales at 4,982 lbs. (2,258 kg), and the gasoline V-8 version weighs in at a bloated 5,330 lbs. (2,418 kg). That is 275 lbs. (125 kg) and 511 lbs. (232 kg) heavier than comparable versions of the Mercedes ML350 and ML550 CUVs.

Despite its heft, the V-8 X5 is plenty fast, hitting 60 mph (97 km/h) in just 6.4 seconds, thanks to the engine’s 350 hp and 350 lb.-ft (474 Nm) of torque. But fuel economy is a dismal 14/19 mpg (17-12 L/100 km).

The gasoline 6-cyl. version is not much better, offering 15/21 mpg (16-11 L/100 km). The 260-hp 3.0L I-6 also feels overmatched most of the time. A midsize CUV hitting 60 mph (97 km/h) in 7.8 seconds isn’t shabby. But because the engine makes only 225 lb.-ft. (305 Nm) of torque at a relatively high 2,750 rpm, delivering lively performance is a strain.

Compare that with the diesel, delivering 75 lb.-ft. (102 Nm) more torque than the V-8 at 1,000 rpms less than the I-6 delivers its meager 225 lb.-ft. The xDrive 35d also does 0-60 mph one second faster and offers 25% better fuel economy than the gas-powered I-6.

That makes diesel a logical compromise between economy and performance.

And this diesel, like its sibling 335d sedan, comes with green credentials. In order to meet tough 50-state emissions requirements in the U.S., the xDrive35d uses a selective catalytic reduction emissions-control system it calls BluePerformance that neutralizes smog-causing oxides of nitrogen by injecting a urea/water solution (labeled AdBlue) into the exhaust stream.

To introduce the SCR technology, which also will be required soon in Europe to meet upcoming Euro6 standards, BMW has developed an active and passive 2-tank system it says improves customer convenience.

’09 BMW X5 xDrive 35d
Vehicle type front-engine, all-wheel-drive, 5-passenger CUV
Engine 3.0L turbocharged I-6 diesel; aluminum block/aluminum head
Power (SAE net) 265 hp @ 4,220 rpm
Torque 425 lb.-ft (576 Nm) @ 1,750 rpm
Compression ratio 16.5:1
Transmission 6-speed automatic
Wheelbase 115.5 ins. (293 cm)
Overall length 191 ins. (485 cm)
Overall height 69.5 ins. (177 cm)
Curb weight NA
Base price $52,025
Fuel economy 19/26 mpg (12.4-9.1 L/100 km)
Competition Mercedes ML320 CDI, Lexus RX 400h
Pros Cons
Tons of torque Noisy idle
26 mpg hwy Expensive fuel
Quiet cruising Complex emissions

Urea is injected from the active tank containing about 1.6 gallons (6 L) by means of a dosage pump. The active tank and dosage pipes are heated because the urea solution freezes at 12° F (-11° C). The active tank is connected to a much larger passive tank that holds 5.7 gallons (21.7 L) more urea.

Due to packaging limitations, the position and location of the AdBlue tank vary from one model to another. In the BMW 335d, the active and passive tanks are at the rear of the car, while in the xDrive35d, the active tank is housed in the front right section of the engine compartment and the passive tank is under the floor next to the transmission.

Having this much urea on board means the system only has to be replenished during regular oil change intervals.

Under normal circumstances, the customer will never have to bother with additional service, BMW says, adding the cost of refilling the tanks is covered under BMW’s no-charge maintenance program for the first four years or 50,000 miles (80,000 km).

BMW says the system will require early replenishment only under extremely aggressive driving. In that case, a warning light comes on about 1,500 miles (2,400 km) before the fluid runs out.

If the driver ignores the warning, only about five starts will be allowed once the fluid gets within 200 miles (322 km) of running dry. But that’s unlikely to happen to a typical driver, BMW emphasizes.

Nevertheless, potential buyers may be frightened off by some other negatives. In the cabin, engine noise is muted and quieter than either gasoline engine while cruising. But a distinctive diesel clatter, which sounds louder than the 335d we also tested, is noticeable outside the vehicle, or when driving at low speeds with the windows down.

In the U.S., it is possible passersby might ask if there is something wrong with the car. That is something a luxury vehicle owner never wants to hear.

The issue of having to hunt for a diesel pump also may concern some potential buyers. But the idea of diesel owners being forced to fill up at truck stops has long since been debunked. Over 40% of filling stations in the U.S. now have at least one diesel pump. In years of testing diesel-powered vehicles in the U.S., Ward’s editors seldom have had trouble locating a diesel pump when needed.

Being able to drive almost 600 miles (966 km) in the xDrive35d on one tank makes it even less of an issue.

All these concerns may only be part of a learning curve. Last year, 71% of all new BMWs delivered to customers in Europe were powered by diesel engines.

After a long day driving the xDrive35d over the Alps from Munich, chugging through small towns and hurtling down the autobahn at triple-digit speeds, we can see why they are so popular.

dwinter@wardsauto.com