When driven back-to-back against the previous model, it's clear the new 2010 RX offers more than cosmetic improvements.Mechanical improvements for the all-new Lexus RX include an independent rear suspension, a 10-airbag safety system, a six-speed sequential transmission, and an improved all-wheel-drive system.Upgraded accommodations include better seats, integrated satellite radio, acoustic glass, automatic high beam control, and a heads-up display among other things.Many of the improvements are due to more and better computer logic for integrated control of driving systems such as the transmission, all-wheel-drive, navigation, steering and brakes.

Perhaps the key interior upgrade is the introduction of a mouse-like controller which Lexus calls Remote touch.It's used instead of a touch screen to operate climate, audio and the navigation system.

Compared to competing SUVs, such as the current Acura MDX, Mercedes ML 350 and BMW X5 3.0, the RX 350 gets better mileage and cleaner emissions ratings.The RX 450h hybrid gets better mileage yet, and is among the cleanest vehicles yet devised by man.

The RX 350 and RX 450h are offered with front-wheel drive or all-wheel drive.The all-wheel-drive system is completely new, lighter and more efficient.It works electronically without the locking center differential in previous models.

The RX 350 comes with a 3.5-liter V6 mated to a six-speed automatic multi-mode transmission.Compared to the current model, the RX 350 is a little heavier due to stouter rear suspension components, which provide upgraded crash performance.

The optional Luxury Package ($4800) includes semi-aniline leather trim interior; one-touch-open/close Moonroof; Power Retractable Electrochromic Outside Mirrors; Wood and Lather Steering Wheel and Shift Knob; Wide Angle Side-View Monitor, 19-inch Alloy wheels with Mud and Snow Tires; MP3 MiniPlug with USB Audio Plug; Power Rear Door; Illuminated Scuff Plate and Headlamp Cleaner.Comfort Package ($2000) includes bi-xenon high intensity discharge (HID) projector headlamps; adaptive front lighting system; automatic high beams; rain-sensing automatic wipers.

Dual-Screen Rear Seat Entertainment with Navigation ($4980) includes DVD rear-seat entertainment system with wireless headphones, audio/video inputs, 120V AC power outlet and navigation system with voice command, backup monitor, XM Nav Traffic and Nav Weather with 90-day trial subscription and Remote Touch controller.

Options include Heads Up Display ($1200), Heated/Ventilated Front Seats ($640); 19-inch Aluminum Alloy Wheels with Mud and Snow Tires ($660); and Bi-Xenon High Intensity discharge Headlamps with AFS and Automatic High Beams ($1175).Mark Levinson Audio ($1610) and Intuitive Parking Assist ($500) are available as stand-alone options.Navigation System ($2400) includes Hard Disc Drive Navigation System with voice command; Backup Monitor; XM Nav Traffic and Nav Weather with 90-day trial subscription; Remote Touch controller.Sports Package ($1300) includes 19-inch Sport finish alloy wheels with mud and snow tires, sport-tuned suspension, Vehicle Dynamic Management (VDIM).

Safety equipment includes front airbags; front-seat mounted side airbags; roll-sensing front and rear side-curtain air bags; driver and front passenger knee airbags and side airbags; active headrests for front seats; force-limiting seatbelt pre-tensioners for driver and front passenger seats; three-point seatbelts with pre-tensioners for all rear seating positions (3); direct-type tire pressure monitoring system; four-sensor, four-channel anti-lock brake system (ABS) with Brake Assist; enhanced VSC (Vehicle Stability Control).Optional safety features include Pre-collision System and Dynamic Radar Cruise Control ($1500); Vehicle Dynamics Integrated Management (VDIM) come with Sports Package.

In the process, the new RX got slipperier, with a coefficient of drag of 0.33, down from 0.35.

The RX has a new keyless entry system that has a better-integrated sensor on the front doors.It looks sleeker, and the door handles themselves have been redesigned for a more comfortable grip.Fog lamps are integrated into the front bumper, and the outside mirrors are heated.

The RX 450h is distinguished by a different, more finely slotted grille and front bumper treatment, and by energy-saving LED headlights that are exclusive to the hybrid.Both the RX 350 and RX 450h have LED brake lights and turn signals, but the hybrid has auto-dimming outside mirrors.The 450h also has blue emblems and logos, and is available in an exclusive White Pearl color.

The RX comes with 18-inch, five-spoke aluminum wheels with 235/60R18 tires; 19-inch wheels are available as part of the Sport Package or as a stand-alone option.

The RX comes to life with the push of a button.The instruments are bright and engaging, and shaded from outside light.The seats now have slightly higher side bolsters for support during cornering, but they're still easy to get into.They feel firm at first, but medium-soft once you settle in, and are easily adjusted to a variety of positions.Mood lighting in the footwells and cabin supports a sense of quality and well-being.

The new cockpit provides a step up in sophistication, conveyed in part by a gifted design scheme that includes no straight lines or sharp corners.Every element of the interior is curved, arched or rounded in some way, so lines blend with subtle grace and harmony.California Walnut wood trim accents are mixed with three different interior color combinations, complementing black, light gray, and parchment tones.

The cockpit is divided into two separate zones.The first is for information functions, located higher up where they are easy to see.The second is for control functions, located lower and within easy reach of both front seat occupants.This division is made possible by Remote Touch.

The Remote Touch system is essentially a computer mouse.It's located exactly where your hand would fall if your elbow were resting on the center console.This mouse will be immediately familiar to anyone who has used a computer.It replaces the need to reach out and touch the screen to use the Navigation system, or adjust audio or climate settings.We found it was simple to use while driving.It allowed us to avoid focusing on the navi screen to tune the climate control or turn down the volume.Plus, no more fingerprints on the screen.

The center console is bridge-like with open storage under the center.The shifter is immediately above the Remote Touch mouse.

There are a number of redundant hard buttons on the steering wheel, and at various points on the dash, 52 in all, but the Remote Touch mouse really does reduce the need to use them.Like a computer mouse, Remote Touch can be adjusted for sensitivity to your preference.

The navigation system is a sixth-generation version, with casual speech voice recognition capability for hands-free operation of audio, climate, and Bluetooth.The system runs from a hard disc, not a DVD, so it's faster and allegedly smarter than the previous system.Among the improvements are easier phonebook transfer, voice command access to the phonebook, and smart auto-fill when selecting destinations.

XM satellite capability is built in, and the first three months are free when you buy the RX.Three services are available: real-time traffic, advance weather forecasts and XM Satellite Radio.It costs $23.99 a month to subscribe to all three services after the trial period.

The standard audio system has 9 speakers, a 6-disc CD changer and can play MP3 and WMA files from your iPod.A 12-speaker system comes with the navigation system, and a 15-speaker Mark Levinson Surround Sound system is available for those who require an audio system consistent with the very best home systems.

The front seats now have longer cushions, with higher backrests, and are fitted with active headrests as standard equipment.The headrests are designed to adjust upward in the event of a rear-end collision, reducing whiplash.Because of the active headrests, rear-seat entertainment screens are independently mounted in pods behind the headrests.The two seven-inch LCD screens can operate separately so one passenger can play a game while another passenger watches a DVD.The seats are covered in fabric in standard trim, with leather and semi-aniline leather offered as available options.

One of our test units had seats covered in the consistently flawless semi-aniline leather, and we were hard pressed to find a misplaced stitch or shadow of stain anywhere.In fact, the leather is so supple and uniformly perfect we first thought it must be synthetic material.Regardless of the covering, both front seats are 10-way adjustable, with memory, and heated/air conditioned seats are available.

The rear seat can accommodate five and has a 40/20/40 fold-down split.The new design makes it easier to release one or both seats from the back, without having to walk around to the side, when loading cargo.

The rear seats also have a fold-down center console with a covered wood grain cupholder and separate covered storage compartment.The rear seats are shorter, with less reclinability than the front seats, but with good legroom for average-size adults like ourselves, much better than a seat on a 757 these days.There are four grab handles, one for each door.There are two rear air conditioning vents to cool the rear compartment.

Cargo room behind the rear seats has been slightly increased to 40 cubic feet, enough for four average golf bags.

When it comes to the interior, the hybrid 450h is essentially the same as the 350, except that the 450h has an Eco driving indicator that displays fuel economy using a bar graph.A hybrid system indicator replaces the tachometer to provide driving feedback.

Out on the road, the big V6 in the RX 350 responds well to throttle.Part of that is the new six-speed transmission, which has a lower first-gear ratio than the old five-speed, so it gets the RX quickly up to speed for merging or highway entry.Lexus told us to expect 0-60 times in the neighborhood of 7.4 seconds, and a quarter mile time of 15.8, and that's about the way it felt.We didn't go there, but top speed is electronically limited to 112 mph.

The transmission quickly selects gears based on throttle input, so you can ask for a gentle downshift just by giving it a little more gas.If you floor the accelerator, it kicks down two gears and moves out.It's also possible to select your own gears in multi-mode shifting, but the automatic's logic seemed so intuitive that we would probably drive in Auto 95 percent of the time, using the manual mode for downhill control on the highway or long uphill when heavily loaded.There is no V8 option for the RX, but if there were, we'd wager that performance would not be much improved, and mileage would suffer.

We did not feel much torque steer in the front-wheel-drive models, and particularly, the hybrid.Full-throttle starts in the previous hybrid elicited a waggle as various components of the powertrain kicked in that we found disconcerting.First, you'd feel the engine, then two electric motors, one after the other.These steering affectations are far better tamed in the new RX 450h.Our assumption is that the transmission's computer is managing torque more progressively, and the flywheel is more heavily damped.

The RX 350 is not built for road racing, but more for passenger comfort.The new rear suspension, a double-wishbone independent, has a lot to do with that.Still, it has a nice, clean turn-in and good lateral grip for an SUV.The new RX is just a little longer than the old model, and stands 2.6 inches wider, so there is a noticeable gain in agility.

We expect Lexus vehicles to be exceptionally quiet, and the new RX is no exception.That said, the 19-inch wheels on the Sport Package do allow for more road input, and a little more noise and vibration seep through.The suspension is tuned slightly differently with the Sport Package, but the end result is still not as aggressive as Porsche's Cayenne or the BMW X5.To us, that's fine.We think SUVs are better at grocery-getting than corner-carving anyway.Bigger wheels, thinner tires and tighter damping tend to compromise Lexus ride quality and noise control, without delivering all that much gain.So our preference is for the 18-inch wheels.

Regardless of the tire choice, we found it easy to maintain a conversation using a low tone of voice at normal speeds.Lexus pioneered the science of quietness in a car, with the result that almost all cars are significantly quieter than they were five years ago.New tricks, such as optimized wheelwell damping coatings and fender liners, have been incorporated into the new RX.The underbody of the car has also received attention against wind noise.And the addition of a sixth gear in the transmission has allowed for quieter cruising, with less engine noise, on the highway.

All those things also hold true for the RX 450h, but because it is a hybrid, it regularly operates with the engine off altogether.During those times, noise reduction is even greater, bordering on eerie.

The brakes feel strong and progressive, with just a little squish at the top of the pedal before stronger grip kicks in.The hybrid brakes, which are regenerative, are now much more smoothly modulated than early hybrid brakes.The transition between light braking, when the generator reclaims power, and the serious stopping power that comes on when more pedal is applied, can still be felt, but you have to look for it.

Steering effort is light, but seems better proportioned than past electric steer-by-wire systems we've tested.It's more accurate than before, especially at the center.The new power-assist logic is based on vehicle speed.The slower you go, the less effort required, so it can be just as appropriate in the parking lot as in the fast lane on the way to Las Vegas.Lexus uses the electric steering system on a number of its vehicles, because it reduces drag on the motor and thus, improves fuel economy.It's also more compact, so there are packaging advantages, and once you have a computer controlling steering, you can add VDIM.

VDIM is not a system easily tested in normal driving, because it requires driving out of control, and then recovering.At which point, nothing will have happened, except maybe a brief flash of an icon on the dash.But it's a system that looks at steering, braking, throttle and motion sensors in the cabin to predict what will actually happen, compared to what the driver is asking for.If there is any difference between the two, the system intervenes and selectively brakes individual wheels to correct the path of the car.It happens so fast you might not notice.

We've tested VDIM on race tracks and lonely dirt roads.It works without slowing down the car much, unlike VSC, which is the standard traction/stability control system.It's an expensive option, but one we wish was on every car.

The RX is an easy SUV to see out of, and fairly easy to keep track of the area around the vehicle when backing up or parking.Driver visibility is enhanced by small windows in the A-pillar, and by small TV cameras that track the rear and passenger-side of the RX.One camera is located in the right side mirror, the other in the rear bumper.We found we could toggle between the two views, front and side, to see how close we were to the curb or pedestrians.

We found the RX to be a relaxed and comfortable ride, so we spent time trying out some of the new electronics systems.

We actuated the heads up display and found it makes it easy to stick to speed limits in unfamiliar ground.Bright white, high-contrast figures were easy to read, even heading into sunlight.It's the best we've seen so far.It's possible to project the display anywhere on the windshield; we liked it best low and to the left.

Voice recognition, something we've never really trusted, still takes a little getting used to, but the new system is much closer to something we might use every day.We began by asking it to switch to channel 143 on XM, which it had no problem doing.It also seemed to quickly be able to supply the nearest gas stations, although not the cheapest prices, and turn down the air conditioning.However, when we asked it to play Grateful Dead, it did not find the channel on its own, so evidently there are limits to Hal's intelligence.

The RX 450h is offered in front-wheel drive or all-wheel drive, but the all-wheel-drive system is different; it incorporates a separate rear electric motor to power the rear wheels.Because it is a hybrid, the transmission is a continuously variable automatic and the brakes are different due to the need to regenerate electricity.And there are a few packaging and styling differences to consider when making a decision between the two.Because of the addition of electric motors, the RX 450h has 20 more horsepower than the RX 350, which compensate for a weight penalty of about 300 pounds.

From a purely economic point of view, the hybrid becomes attractive if you believe the cost of fuel will again rise sharply.The hybrid RX gets about 7 mpg better fuel economy, so it goes 134 miles farther on a tank of gas.That translates into a savings of $12.80 per tank if gas stays at $2 per gallon, or $25 per fill-up if gas goes back up to $4 per gallon.If the premium for the hybrid system is $4000, you would have to drive 83,700 miles before the cost is amortized.Over 100,000 miles, you would burn almost 1200 gallons less gas, but save only $786.

In short, dollar savings would be modest, even if prices rise.For that reason, most current hybrid buyers chose the vehicle because they want to drive the most fuel-efficient, cleanest SUV possible, one that reduces dependency on imported oil.The new RX450h takes that reasoning further, delivering up to 12 percent better fuel economy and 27 more system horsepower than the current-generation RX 400h hybrid.Total emissions are approximately the same as the current RX400h, even though the 450h has a bigger engine and more power.Which is to say extremely low.

The RX 450h we drove averaged 23.4 mpg over 120 miles of mostly around-town driving.

The hybrid brake system uses the same ventilated four-wheel discs in the RX 350, but has an electronically controlled regenerative brake feature that charges the batteries when the brakes are applied gently.The hybrid system includes ABS, Electronic Brake-force Distribution (EBD), Brake Assist and more important, Vehicle Dynamics Integrated Management system (VDIM) as standard equipment.We've tested VDIM on controlled proving-grounds settings, and consider it a remarkable safety system.It's also available on the RX 350 as an option.

The hybrid can drive in three modes: Normal, Eco for best mileage, and EV Mode which allows the vehicle to operate on battery only.Lexus tech sources estimate the EV-only mode operating distance at about a mile.The EV feature is aimed at allowing the RX to be used as an electric car in closed urban areas where internal combustion engines are banned altogether, such as in Europe.

John Stewart filed this report to NewCarTestDrive.com from Sonoma, California.