Twist the key and everything on the dash lights up, but there's no sound of an engine starting, only silence. Slide the transmission lever to drive and you can pull silently away on electric power. This is not at all intuitive at first because we're all accustomed to hearing and feeling an engine running before shifting into drive. Shifting into drive when there's no sound, no vibration is a new experience.

At low speeds, the RX 400h is perfectly content to operate in electric-only mode. It's quiet, and you begin to hear things that are normally drowned out by an engine. We found this interesting and enjoyable. It will run in this silent mode in stop-and-go commuter traffic, eliminating the noise and pollution that the cars around you are putting out. The RX 400h maneuvers silently through crowded parking lots as well, where pedestrians often will not hear you coming and therefore won't always get out of the way. Toyota's hybrid system is an electric motor assisted by a gas engine, while Honda's system is a gas engine assisted by an electric motor.

The gas engine starts whenever it's needed to supplement the electric motor. Step hard on the gas pedal and the V6 kicks in quickly and seamlessly. You can barely hear or feel when this happens. It works exceptionally well. All the components work in concert most of the time.

The 3.3-liter V6 is basically the same engine as the one used in the RX 330, but it's modified and tuned to work with the electric motor. It's set up for the on-demand instant restarting used by the hybrid system. Unlike the RX 330's engine, the engine in the hybrid does not use the variable-valve setup because it's not needed and takes up space. There's no starter, no alternator, and no serpentine belt to run all that stuff.

The RX 400h enjoys a 67-percent edge in fuel efficiency over the RX 330 in the EPA's City cycle, earning 30/26 mpg City/Highway. Its 28 mpg EPA Combined rating is comparable to that of the average compact sedan (27.6 mpg); average for SUVs in this class is 15.1 mpg. Lexus says fuel economy is seldom an important consideration for SUV shoppers but that many buyers later become unhappy with the thirstiness of their SUVs after owning them.

More impressive is the absence of environmentally harmful emissions. The RX 400h is rated as a Super Ultra Low Emission Vehicle in California or Tier 2, Bin 3 in other states, confusing designations that describe some of the most stringent emissions ratings in the industry.

But what surprised and delighted us was the acceleration performance of the RX 400h. Stand on the gas and it takes off quickly, whether from a standstill or when cruising. Lexus says the RX 400h can accelerate from 0-60 mph in 7.3 seconds. That's a half-second quicker than an RX 330 and comparable to the Mercedes ML500, which uses a thirsty V8. Lexus says the RX 400h powertrain can generate 3,500 pound-feet of torque at launch.

The RX 400h cruises happily at speed out on the highway and there's plenty of reserve power on tap. When we caught a slower car on a two-lane road, we checked to see it was clear, pressed the accelerator to the floor, and were able to make a quick and easy pass. Safe, quick passing performance was important on Hawaii's Big Island where passing opportunities were limited. Lexus says it takes the RX 400h just 3.4 seconds to accelerate from 30 to 50 mph, at least a solid second quicker than the competition. We were sometimes able to catch the system off guard, however. While coasting down a hill we suddenly nailed the throttle, not a move the car seemed to expect, and there was a momentary lag before the power kicked in. For the most part, though, the RX 400h drives like a powerful and sophisticated SUV.

If we were to nitpick, we'd say it drones a little under light throttle and sound it makes under acceleration isn't entirely pleasing. Also, there's a small amount of torque steer, or something that feels similar to torque steer: Pull up to a stop sign, stand on the gas and make a right turn, and you'll feel a slight tug on the steering wheel or a resistance to turning. But all this amounts to nitpicking and the description of it sounds much worse than the reality.

The RX 400h glides along like it's on a cloud on smooth pavement. The ride is firm but damped. You know when you're on a rough road but the ride is not harsh. The electric steering works perfectly. The steering system uses electrical signals rather than hydraulic fluid and is speed-sensitive, boosting power assist at low speeds for effortless parking. The RX 400h felt composed and responsive on narrow, winding, bumpy roads in the back country high above the Kohala Coast.

The RX 400h was not designed for serious off-road use. The regular RX 330 isn't exactly an off-road vehicle, either, but it handles gravel roads just fine. The RX 400h, on the other hand, should stick to the pavement or risk damaging the hybrid drivetrain.

Slippery surfaces, however, are no problem. The electronically controlled all-wheel-drive system should work very well on wet pavement, snow and ice. One of the three electric motor generators (known as MGR) drives the rear wheels whenever they're needed for improved traction. This electric motor can deliver up to 650 pound-feet of torque to the rear wheels at launch.