Under the hood of the GLS63 is a twin-turbocharged 4.0-liter V-8 engine that makes a whopping 603 horsepower with the help of a 48-volt hybrid system. All-wheel drive is standard, as is a nine-speed automatic transmission and air suspension. We were blown away by the 2021 GLS63’s performance at our test track as it out-accelerated several hot sports cars we’ve tested recently, tearing off a blistering 3.6-second zero-to-60-mph run. For comparison, the old GLS63—which made a mere 577 horsepower—managed a none-too-shabby 4.3-second zero-to-60-mph run. As impressed as we were with the GLS’s acceleration, we can’t say the same for its driving dynamics. The ride is firm, especially on the optional 23-inch wheels, but that firmness doesn’t translate to a feeling of precision when cornering. The GLS’s body bobs across road undulations more than expected, though it still offers plenty of grip to have some fun with on a twisty road.