generally doesn't do instrumented performance testing on new vehicles, but Car and Driver has done just that for decades. Last year BMW pulled the covers from its mildly updated 5 Series, including the snazzy M550i xDrive packing a 523-bhp (390-kilowatt) twin-turbocharged V8 under the bonnet. BMW says it reaches 60 mph in a scant 3.6 seconds, but the C&D team found otherwise. Eventually, so did BMW. More on that in a bit.

Last year, tests by the magazine could only muster a 4.1-second sprint to 60 for the beefy Bimmer. Obviously there are all kinds of factors that can affect performance but a half-second is a bit difficult to quantify. To follow up on that, a second series of tests were conducted that saw the time drop to 3.9 seconds. It's a small margin for sure, but we give credit where credit is due. The enduring magazine has a good rep for pulling strong performance stats so they asked BMW to send them a different M550i for testing. That didn't happen, but not for nefarious reasons.

In the process of all this, it seems BMW uncovered a minor technical glitch that affects both the M550i and the 540i xDrive models. According to Car and Driver, a communications error between the car's engine and stability management systems causes a small loss of boost pressure when given the beans. All the horsepower is apparently there, it's just literally getting lost in translation. As a result, the cars don't have quite the punch off the line as they should have. Not that nipping 60 mph in under four seconds in anything is slow, never mind a sizeable saloon.

It's not clear if BMW was aware of an issue before the testing, but an over-the-air software update to fix the issue is reportedly coming at some point this summer. has contacted BMW for official confirmation and additional information on this, and we'll plug in an update should fresh information become available.