The S400 Hybrid does not meet some definitions of hybrid since it will not propel itself on electric-power alone.   However, it does increase fuel mileage more than 25 percent in the city and to a rated 26 on the highway (we recorded 21.7 around town and 27.6 on the highway) with no downside or price premium; the battery is in the engine compartment and the S400 weighs only 19 pounds more than S550.   With a combined power output of 295 hp and broad torque its acceleration betters most hybrids but an S550 is quicker, yet an S550 invokes traction control trying to use all its power from a standing start anyway.  

Five characteristics segregate S400 from S550: The S400 Hybrid makes a different noise, not rougher or louder, merely different.   It often switches off automatically when stopped to save fuel so the tachometer swings to zero; taking your foot off the brake or touching the gas pedal restarts it, and it makes this transition smoother than any hybrid we can think of, including pricier V8s.   In places where you are going very slowly, as you might creeping into a tight parking spot, the idle stop may be more active than you like so just resting your big toe on the gas pedal to keep the gas engine running will smooth things nicely.   The brake pedal has more of an on/off switch feel to it because that's a lot of what it does, but if you hit it the S400 will stop in short drama-free order.   Finally, powertrain/battery status is added to the various display options.  

Ride quality is superb, S-Class air suspension combining smoothness with complete control and utter stability as you waft along faster than you think.   We found it duly goes where it's pointed and if you think yourself getting into an off-ramp a bit too fast you'll be impressed by what this 4,500-pound mass can do even before any electronics come into play to save you from your own poor driving habits.   The suspension can be raised at slow speeds for excessive speed bumps or driveway angles, lowers at higher speeds for stability and economy, and can be firmed up in Sport mode if you prefer quicker reactions to pillow-gentle manners.  

Steering inputs are fluid, linear, predictable and surprisingly crisp for such a long wheelbase; on some models a quick turn puts a touch of brake to a rear wheel to help encourage the turn but there is no artificial feel introduced when it happens.   Brakes are easily modulated and seem endless in their ability to retard harder as you further depress the pedal.  

An S550 behaves essentially the same way except for more significant thrust by virtue of its 382 horses and 391 lb-ft of torque, nearly 300 of which is available at just 1000 rpm.   S-Class cars start in second gear to save fuel (unless you've chosen Sport or Manual modes), but even when starting in second gear we found the S550 has considerable urge.   The 4MATIC model gets going more easily in poor conditions.  

Active Body Control (ABC) adds another element to suspension control by mechanically countering the acts of physics.   As the steering wheel is rotated to make a bend the car automatically alters suspension to remain flatter.   If you've ever watched a motor race where the driver swerves back and forth to warm or clean tires but the car appears to lean very little, ABC gives the same effect to a much heavier, taller, softer riding car.   BMW's 7 Series offer active suspension as well but they lack the linear, more organic feel of the S-Class.  

The S600, with a twin-turbo V12 engine, whirs and hums rather than starts and runs, with a fluidity matched only by more-expensive twelve-cylinder cars.   With 510 hp and a prodigious 612 lb-ft of torque (at just 1800 rpm) the S600 gets a five-speed automatic because the seven-speed automatic can't handle it.   The S600 will run 0-60 in less than five seconds with four people on board as long as you can find traction.   S600 uses 18-inch wheels like lesser S-Class but they're wider in back to help cope with the power.   We found the S600 has such ample reserves of power at any speed that the gas pedal should be treated as such.  

The S63 AMG is rated at 518 hp, but it is a different breed than the S600.   With a hand-built 6.2-liter V8 that burbles and bristles like a refined muscle-car, it revs to 7200 rpm.   With a mere 465 lb-ft of torque, the S63 uses an AMG-modified version of the seven-speed automatic transmission.   It matches the S600 for speed but has crisper, racier response, although the seven-speed didn't seem any quicker shifting to us than older AMG gearboxes.   Along with the biggest V8 from Germany come massive brakes, AMG-calibrated ABC suspension, 20-inch wheels and ultra-low profile tires, and every component is designed to maintain a torrid pace.   The S63 is not the fastest S-Class but it is the most driver-oriented and the most sporting.  

The S65 AMG is a wolf in sheep's clothing that marries the leather-and-suede luxury of an S600 with the sporting chassis of an S63 to a 6-liter twin-turbo V12 and SpeedShift five-speed automatic.   It generates 604 horsepower and a staggering 738 lb-ft of torque at 2000 rpm as smoothly as a jet engine, making your head the nail, the headrest a center-punch and your right foot the hammer.   With more torque than any diesel pickup and twice the horsepower of a typical sport sedan an S65 with traction control off can spin tires through 70 mph and is electronically limited to three-miles-per-minute top speed.   It will accelerate ferociously from 60 mph to 120 faster than most cars will from stop to 60, yet is easily managed if you don't switch too many things off and we found it downright docile when driven moderately.   Effortless is a wholly appropriate descriptor here.  

Every S-Class driver has a quiet cabin to work with virtually free of wind noise to freeway speeds and normal-volume conversations (with your driving instructor) can be maintained at 130 mph.   Road noise increases nominally with larger wheels and still won't be heard above a talk-radio program, and engine noise is greater in the AMG models but either will cruise in subdued tones.