ONLY a practised eye will spot the differences between the new model Gran Turismo and last year's car, but even these slight changes have transformed the machine. The Gran Turismo is now the sporting saloon with driver appeal that Nissan needed to complete its line of big luxury cars. Nissan has dropped the ride height only five millimetres and added different alloy wheels for mechanical changes, but the Gran Turismo feels much more secure on the road. It seems more like the sporting car Nissan set out to build in the first place. The first test of the Gran Turismo in the Motoring Post of July last year praised the comfort of the car, but said: ''Pushing hard through the corners has the body rolling and control becomes ragged as the weight of the car moves unchecked.'' The new car displays better body control and inspires confidence in the corners where the older model felt nervous and less stable. Comfort has been preserved and the big car moves along the expressway with the plush ride of a limousine. This is hardly surprising, considering the Gran Turismo remains similar to the Cedric, sharing the majority of its components with its luxurious sister. The two cars share the same chassis and the engine is the same three-litre, V-6, 200 brake horsepower unit, and all the major body panels are the same. But detail changes make the cars appear different. Four round headlights are substituted for the Cedric's two rectangular lights, and a deeper chin spoiler adds an aggressive look to the Gran Turismo. The 1993 model features new-style wheels and slight alterations to the arrangement of the lights with the indicators moving down from besides the headlights into the spoiler. The Gran Turismo now looks much leaner than the Cedric. The interior is almost the same subdued cabin as last year, but hidden within the dash is an important safety feature. A passenger-side airbag is neatly stashed, complementing the tidy installation of the driver's side airbag mounted in the boss of the steering wheel. Safety is further enhanced with the inclusion of pre-tensioner front seat belts and a limited slip differential helps keep the car in line in slippery conditions. The Gran Turismo is meant to be a little more spartan than the Cedric with emphasis on driving pleasure rather than comfort, but the cabin is still extremely well designed and finished. Full ''opera lighting'' illuminates all the corners of the cabin, including the crannies where coins are lost and skirts are snagged. Each doorway is lit up when the door is opened and a spotlight is on the driver's side. The wood trim is a little darker for the 1993 Gran Turismo and the leather upholstery is soft and high quality. Ashtrays are well built in wood and chromed metal that fold away out of sight, and clever touches, like the folding of the mirrors at the touch of a button, show that the car is a product of thoughtful design. The line of the dash and the bonnet is quite high, making the car seem large and tall from the driver's seat, but as the kilometres slip by this transforms into a feeling of security. The driver and passengers feel protected by the high sides of the vehicle and the apparent solid construction. The V-6 engine is an enthusiastic motor, which accelerates well no matter what the ratio the automatic gearbox has selected. Noise is limited to a gentle growl despite the brisk performance of the Gran Turismo. The price of the new Gran Turismo is $465,000, which is probably a little less than the yet-to-be-announced price of the improved Cedric.