Speaking of stops, the CL63 incorporates an auto stop/start function-a first for Mercedes (outside of its hybrids). It should give a boost to real-world fuel economy, and indeed, preliminary EPA estimates for the CL63 are 15/21 mpg, significantly better than the 2010 model's dismal 11/18 rating. (The CL550's figures improve by 1 mpg both in city and highway driving, to 15/22 mpg.) The auto stop/start function works well, restarting quickly enough so as not to impede takeoff. But it's strange how the sudden silence at red lights actually calls attention to itself-expect conversation to stop as well. The auto-stop feature works only if the transmission is in Comfort mode (which also makes for earlier upshifts and second-gear starts); in the Sport and Manual modes there is no engine shutoff.
Although the CL63's twin-turbo 5.5-liter outmuscles its big-block predecessor, don't expect the sharp staccato bark that we know-and love-from the 6.2-liter-and-Speedshift combo in the E63 and SL63 AMG cars. The CL's rumble is a much more subtle baritone. The CL550's 4.7-liter V-8 is quieter, enough that you can sometimes hear the whistle of the turbochargers. With its new engine, the CL550 is now nearly as quick as its AMG counterpart, its factory-measured 0-to-60-mph time having dropped from 5.4 seconds to 4.8. That's only 0.4 second behind the new CL63, or 0.5 second slower than the performance-enhanced version. What this means is that, when you bury the long-travel accelerator in either CL, you're instantly beamed to the bumper of the car ahead.
The more muscular V-8s pretty much render the V-12 engines superfluous, but they remain in the lineup. The CL600 retains its twin-turbo 5.5-liter twelve (510 hp, 612 lb-ft). The CL65 AMG's biturbo 6.0-liter V-12 sees a minor power increase (now 621 hp and 738 lb-ft). Still, the CL65 can only tie the 563-hp CL63's60-mph sprint, likely due to traction limitations. The V-12 cars are available by special order only.