I would go with a dress and flats :),
Shorts are nice but everyone will be wearing them. I've been to a ton of car shows and they seem to be almost a uniform at a lot of shows. Heels can get a little uncomfortable if you end up staying there all day, although maybe you can go in a pair of heels and bring a pair of ballet flats in your purse just in case? helps in case you want an awesome photo with a car!
If you bring a parasol it helps too!
I agree with both Brandy's... usually, wearing my petticoat under my dress allows for a little extra "air conditioning" when there is a nice breeze (or when you're walking, lol). And definitely bring a back up pair of shoes - you may start out in your most comfortable wedges that you swear by, but with heat & humidity (and any car show junk food available!), swelling feet does happen!
Have fun & post pics!!
If it is outside I go with an adorable shirt and a cute vintage hat. Wear shoes that you can walk around in and still feel comfortable and stylish. Car shows have all kinds of outfits, but you don't need to over dress.
I love a themed show and get lots of comments on the ones I have that are car themed. http://thegildedtroll.storenvy.com/
I have one with my husbands Morgan on it and another with his Ferrari. Always gets a ton of compliments and I am comfortable and chic.
I agree that flats or a pair of sneakers an an absolute must to wear for walking around, but a pair of heels in your purse might be good to pull out and put on in a few pictures. Maybe even an extra pair of flats or sneakers in case you wear out the ones that you came with in your purse is a good idea too, like Brandy had mentioned.
A parasol will also come in handy and there are a hundred things that you can do with it when it comes to posing.
Other than that it's honestly all up to you what to wear, although if I had to choose a look I might go with Brandi's idea of an airy swing skirt or dress with a petticoat underneath like Desilu had said as well :).
Skirts and cute dresses are great if you are inside and not spending a long day at the show. The ones I go to are outside and we walk hills and fields looking at the cars and the races. Often I am sitting in a folding chair or on the grass, so pants are a much better option for me. Heels are not good as you would sink into the dirt or have trouble walking long distances.
Love the parasol idea as the show I went to last weekend was well into the 90s and a little shade was appreciated.