A lot of people like to wear "period" outfits in keeping with the cars but I don't think a tie is a requirement.
Goodwood is on my list of things to do before i go for the Big Sleep
This is from their web site:
"What should I wear?
As the Festival is an open air event staged primarily in parkland, comfortable shoes and casual clothing is best. Always bring some light wet weather clothing as, even in summer, British weather can be unpredictable! If you will be using our corporate hospitality pavilions, then smart casual dress is required."
