Truth be told, it's a charging port.
If you assumed the Bentley Bentayga plug-in hybrid would be the only Bentley to offer this powertrain tech, then you assumed wrong. Our spy photographers caught this Bentley Flying Spur prototype doing some late winter testing and noticed something interesting about it: an extra "fuel cap" for the charging port in the left rear fender. What does this signify? A plug-in hybrid version. The next generation Flying Spur has been caught testing previously ahead of its expected late 2018 or early 2019 premiere.
It's entirely possible the plug-in hybrid version will debut at the same time. And because Bentley is part of the Volkswagen Group, the Flying Spur plug-in hybrid will share its guts with not only the Bentayga plug-in, but also the Porsche Panamera 4 E-Hybrid. This means there will be a turbocharged V6 gasoline engine paired with an electric motor. The Porsche has a combined output of 462 hp, so expect something similar in the Flying Spur. However, and this suggestion should not be ruled out, it's also entirely possible the Flying Spur plug-in hybrid has the powertrain from Panamera Turbo S E-Hybrid, meaning a combined 680 hp. Why would Bentley opt for the more powerful system?
Because it may want the plug-in version to be the most powerful and expensive Flying Spur in the range. It's also worth pointing out both the Flying Spur and Panamera share the same platform, which can be adapted for rear- and all-wheel drive. We also suspect the Bentley will be slightly longer than the Porsche, meaning the platform was designed from the get-go for some flexibility.