PARIS – Renault SA’s Nepta concept car for the Paris auto show this month says nothing about the serious possibility of a gull-wing door and everything about the probability of using Nissan Motor Co. Ltd.’s Infiniti luxury brand rear-wheel-drive platform one day.

Nepta, a name meant to recall Neptune and underline the design’s sailboat cues, is a 4-seat convertible the size of a Mercedes-Benz S-Class.

Everything except the floor is creamy leather. Seats are fixed to the floor with mast-like aluminum feet, and the carbon fiber body lets the 420-hp 3.5L bi-turbo V-6 push the vehicle 62 mph (100 km/h) in a theoretical 4.9 seconds.

Like most Renault concept cars, this is a real vehicle, but no one dares drive it to its theoretical limits.

“Nepta makes reference to car styles of the ’30s and ’40s,” says Patrick le Quement, director of industrial design, “but it is not retro. It is classic and modern.”

Le Quement says Renault is working on proportions for its future luxury cars, noting the long rear overhang is a hallmark of rear-drive cars.

The center drive-shaft tunnel is about 6 ins. (15 cm) higher than it needs to be, containing storage space but also assuring that anyone looking at it knows it is RWD, as are all the big Mercedes, Audi, BMW, Lexus and Infiniti cars and unlike the unsuccessful Renault Vel Satis.

Nepta is the first RWD concept in modern Renault history, at least for a concept shown to the public.

Renault CEO Carlos Ghosn promised last winter the auto maker would develop a half dozen new vehicles for luxury segments. Le Quement smiled as he says, “It’s fun to work in an environment where we are developing many new vehicles.”

Although the Nepta is a convertible concept, technicians didn’t bother to work out how a cloth top would fold up and down; it doesn’t exist.

The carbon fiber doors, weighing slightly less than 88 lbs. (40 kg) each, are motorized and fastened at each end with a stainless steel latch. But because they are purely conceptual, there is no side-crash protection.

The seats are fixed, so the steering wheel and pedals adjust to the driver. The third pedal is a footrest, not a clutch. The 7-speed automated transmission is controlled with steering-wheel paddles.

The Nepta is full of show-car gimmicks to please a rich buyer. The gas tank opening is hidden by the high-mounted third taillight that looks like a sail.

The instrument panel has what may be the first barometer built into a car, sharing space in the middle of the dashboard with an analog thermometer and altimeter. The dash is free of switches, but the steering-wheel column bristles with them.

The seats are beautiful, but they don’t adjust up or down and the back angle can’t be changed. However, the serigraphic printing on the leather covers is a real, industrial idea. Le Quement says printed or branded leather is a trend gathering speed.

Fancy lighting developed by Valeo SA also is coming quickly into the market. The external lighting uses light-emitting diodes and light guides, technologies that already are in production elsewhere. However, the headlights use what Valeo calls Total Internal Reflection, in which light from LEDs is sent through a block of crystal.

The car is the last concept designed by Michel Jardin, who ran Renault’s concept studio for a decade before retiring in July to pursue his interests in music and sailing. He is replaced by Stephane Janin, who previously worked at Renault-Samsung Motors Inc. in South Korea.

As Le Quement poses in the sumptuous Nepta’s driver’s seat for photographers, he comments, “I feel like Harley Earl,” the legendary General Motors Corp. designer credited with inventing the idea of concept cars.

Renault will present the Nepta to the public Sept. 28 at the Mondial de l’Automobile in Paris.