Several stage musicals call for a car on stage, especially "Grease," where the car is the centre of much of the action. An important consideration is how real the car needs to be. If actors do not need to actually sit inside the car, or if the car does not have to roll across the stage, you can often get by with plausible substitutes short of building a whole car.
Do not use a real car. Real cars, even those retrieved from scrapyards, present too many hazards on stage. Petrol tanks and other parts will contain flammable residues that could ignite and cause deadly backstage fires. Real cars are far too heavy and difficult to manoeuvre. Never drive a real car onto a stage, regardless of size.
Determine if a representation of a car will suffice. Some musicals, such as "Gypsy," "Working," "Annie," and "Grease," can tolerate cartoon cutouts of cars because of the whimsical nature of the show. Cartoon cutouts can be made from tempered hardboard, Hollywood type rigid flats, or 15 cm (1/2 foot) styrofoam sheets. Trace the shape of the car onto the material and cut out the shape with a power sabre saw. Paint to suit. Attach sturdy handles to the back of the cut out.
Consider "miming" the car. Stage shows, such as "Driving Miss Daisy", use wholly imaginary cars. Four chairs, plain in design, can provide the car seats and the actor in the front seat can mime driving motions and activities. Cars can also be only partially imaginary: Mount real car seats on rolling platforms and push them into place. Let the actors mime driving and getting in and out of imaginary doors.
Build a working replica of a car. Design a convertible model to avoid having to construct a roof and working doors. Build a rolling stage platform equal in size to a car. Use air casters or industrial-sized 360-degree swivel casters if the car has to move across the stage in more than one direction. Construct car chassis framing on top of this platform with 2.5 by 7.5 cm (1 by 3 inch) timber. Face the framing with flexible 3 mm (1/8 inch) thick tempered hardboard or fibreglass. Make wheels carved from rigid urethane foam. Install a plywood dashboard and thread a facsimile of a steering wheel through the dashboard. Mount real car seats inside the framing. Provide for a hidden or upstage entry and exit slot for actors to pass through. Add lots of trims and other accessories to "sell" the idea. Paint the car to suit.
Construct a large turntable to move a large stationary prop car off the stage quickly. Mount the car on the turntable. Build a screen or a "garage" on the stage so that when the turntable rotates, the car disappears inside the garage. An alternative is to mount the car on a sled connected to a floor track. Move the car forward onto the stage and back again into the garage. Operate either turntables or sleds manually with stagehands.
Build car chassis representations over operating electric golf carts using wire framing and 1.2 cm (1/2 inch) thick styrofoam sheathing, fibreglass, or chicken wire and papier mache.