BMW claims the 2021 M550i xDrive does 0 to 60 mph in 3.6 seconds, but when Car and Driver tested it several months ago, the sports sedan was much slower. The sprint took a disappointingly high 4.1 seconds, which was two-tenths of a second slower than C&D’s X5 M50i test, an SUV with the same engine that carries around no fewer than 856 pounds (388 kilograms) over the M Performance 5 Series model.

So, what gives? Some would be tempted to say there was something wrong with that specific car, be it the engine, tires, gearbox, traction control, or the all-wheel-drive system. After a while, C&D got in touch with BMW USA to redo the test and see if they would be able to improve the sprint time. They were, but hitting 60 mph from a standstill took 3.9 seconds, so still three-tenths of a second more than what’s written in the specs sheet. Typically, the real-life tests conducted by Car and Driver result in quicker acceleration than what a manufacturer claims, especially when dealing with German brands. These are known to deliberately underrate the performance of their sporty cars.

During the second attempt, the mystery was solved by BMW’s engineers, finding a software glitch to be the culprit for the M550i xDrive’s unusually slow start off the line. Specifically, it was a “miscommunication between the engine computer and the car’s stability-control module,” reducing some of the V8’s power in a hard launch from 0 mph. The same issue plagues the 540i xDrive, also from the 2021 model year.

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This past summer, BMW informed owners of the 2021 M550i xDrive that a fix for the acceleration problem would arrive later. That day has come as the software error can now be fixed either through an over-the-air update or taking the car to a service center. C&D was obviously curious to find out whether the issue has been solved, so they borrowed another press car to perform a third and final test.

After unlocking all of the 523 horses generated by the twin-turbo, 4.4-liter engine, the “M5 Lite” ran the sprint to 60 mph just three and a half seconds, thus making it a tenth of a second than what BMW says. During the test, it completed the quarter mile in 11.8 seconds at a trap speed of 119 mph (192 km/h).

For an even quicker 5er, there are three hotter derivatives above the M550i xDrive, the M5 and M5 Competition as permanent members of the lineup, plus the flagship M5 CS available only for the 2022 model year as the brand’s most powerful road car ever.

[Source: Car and Driver]