THE Lexus LS400 has been improved for 1993. Described as the ''quality benchmark'' for luxury cars by the American Consumer Reports magazine and voted the best Japanese luxury car by Hongkong's Automobile, the Lexus was hardly in need of revision, but the93 version comes with better equipment. The changes are small but significant. Wider tyres of a lower profile fill up the wheel arches and the body mouldings, such as the skirt beneath the rear bumper, are larger and coloured to match the paint work. The brake discs are an inch bigger to improve stopping power. The airbag safety system, which inflates to cushion the driver in the event of a collision, is now duplicated on the passenger's side, to give protection to both front seat occupants. The front seat passenger can now enjoy the same electrical adjustment of the chair as the driver, including lumbar support and height variation. Other changes are small but the list is long. Among the additions for 1993, the car is fitted with an outside air temperature sensor, a window washer tank level indicator and an electronic odometer. In addition, the seat belt buckles are lit for convenience. What has not changed is the car's supremely high quality and hushed interior. The Lexus LS400 set new standards in the luxury class at a price that the traditional European luxury car marques found difficult to match. Crown Motors have given the LS400 saturation advertising coverage, but, for once, the hype is well-founded. The most outstanding feature is the profound quiet, within the cabin. There is a church-like calm and near-silence that has the passengers talking in whispers. The rush of wind and tyre noise fails to penetrate the Lexus sound-proofing, only the sound of the transmission at high revs intrudes. On the crowded go-nowhere roads of Hongkong, the Lexus provides a sanctuary of peace. The Compact-Disc system is of superb quality and some Lexus owners are said to retire to their cars to enjoy music in the evenings rather than relax in their own homes. Lexus has fitted some extra-luxury touches to the LS400 to further coddle the occupants. The gear shift knob is now covered in leather instead of urethane, and the front header trim and sun-visors are now covered in fabric. But there is surely room for a higher specification LS400, perhaps a special edition featuring the highest grade of soft leather and better use of wood trim inserts. Performance from the big V-8 engine is beyond criticism. As a luxury express, the car is superb. Overtaking manoeuvres are almost ridiculously easy, the big saloon flinging its bulk past slower traffic. More precision from the steering would be welcome. At first, it feels almost unnervingly light. However, the 93 revisions confirm the status of the LS400 as, perhaps, the world's finest saloon, irrespective of price.