BMW's success with diesel engines has put the maxim that American's just don't get the oily fuel to the test: the X5 35d and 335d have sold remarkably well, constituting about 28 percent and 6 percent of total model sales, respectively. So the latest report that the 5-Series may get a diesel variant here in the U.S. makes sense, both from a performance/efficiency standpoint, and as a business case.The larger the vehicle, the more diesel makes sense, after all, delivering high torque at low rpms, making for a more responsive and efficient powertrain than even turbocharged gasoline engines. And as we had the chance to drive the 2011 535i with the new twin-scroll single-turbo N55 engine this week (a more detailed report is on its way) we can attest that more torque and similar power figures will be a good combination in the platform.While the new N55 engine is rated at 300 horsepower and 300 pound-feet of torque, the diesel six, also found in the 335d, is rated at 265 horsepower and 425 pound-feet of torque. The narrower rev range where power is produced, a common trait of alldiesel engines, ought to pair nicely with BMW's new 8-speed automatic transmission, as well. We preferred the six-speed manual in the 2011 535i, but that may not be an option for the potential 535d.All of this assumes BMW will simply stick the already-homologated diesel from the X5 and 335 into the 5-Series, of course. We haven't heard anything from within BMW on plans to homologate the 40d line of diesels for the U.S., however, so the only other likely alternative would be one of BMW's four-cylinder diesels. That would almost certainly prove too little power forAmerican tastes, however, though such an engine might soon find its way into U.S.-market 3-Series cars. That leaves the familiar six the best bet in our eyes, and that's something we can get behind.
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