The Lotus M90 (sometimes referred to as the Lotus X100) is a concept car that was developed by Lotus using many Toyota parts. Rumors were circulated that it was the initial design for the MKI MR2, but, though sharing an engine, it bears very little resemblance to the mid-engine roadster.

In 1981, Oliver Winterbottom was brought back to Lotus, after a stint developing the Tasmin for TVR, to design a new car to complement the Excel and the Esprit and bring some profitability to the lineup. The new car was to resurrect the Elan name which had not been used since 1975 and, like the Excel, make use of the company's close relationship with Toyota to use already well engineered suspension and engine components. Initial designs were a coupe that didn't get much of an enthusiastic response from the board of the company. The project stalled when company founder Colin Chapman died in 1982, as Lotus found itself in financial trouble. David Wickens of British Car Auctions ended up taking a controlling interest in the company, and the project was started up again, but renamed to the X100 (the Elan later being known as the M100). After redesigning the car to be a convertible the go ahead was given in 1984 to make a single prototype. It was built to a high level of finish, but by that time most of the company's engineers were concentrating on the new front-wheel-drive Elan, and the prototype was warehoused at the company's factory. In 1998 it was auctioned off along with several other rare Lotus cars, and sold to a private collector, and shipped to the United States.

The M90 makes use of a front-mounted 1.6L 123 bhp (92 kW) 4-cylinder Toyota engine, as well as a Toyota-built 5-speed transmission. The suspension is also from Toyota, using the same setup as is found in the Excel. The instrument cluster is from an Opel Monza and the radiator is from an Austin Allegro. The tail lights are borrowed from the Aston Martin Lagonda and the turn signal indicators are from the Ford Fiesta. The body is made of fiberglass and the roof is lowered in two pieces. The top is removed separately and fits in the trunk, to create a 'targa' style roof, and the rear portion folds down like a typical convertible.

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