Wow have not done that in a while, but it helps if you have an assistant. 24mm lens is always good to show the model and the car. If you have the room you can go to a long lens, keep in mind that the long lens will compress the car as well.
Wear a black shirt that way you don't show up in the chrome.
Have a flash or two to light the inside or under the car as well as fill for the model.
Most of the owners will work with you if you are at a show, take advantage of the interior of the car as well.
Get there a little early so you don't have a busy background with lots of people. Most of the cars will be getting readied for display with trophies etc. so if you get there just a little before start of show you may get some shots otherwise missed.
Take something to write on etc. to take owner information if it is for a publication. Most of the cars will have the entry form on them somewhere that you can photograph before or after.
If the show is indoors read up on dragging the shutter.
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Shoot well, Joe