Perhaps rather than advertising how much business you have done, you can annually celebrate your wash's birthday. Like "Happy Birthday Joe's Car Wash. 20 years and still washing." Something to that effect.
Offer discounts, raffled prizes, free washes, anything you can muster up. It's your birthday, but you give the customer the presents for keeping you alive so long. If you want to do something bigger on the "big ones" like every five or ten years, I'm sure you can think something up.
Washing your 1,000,000 car is great, but it is essentially opening up your accounting book to the public - not a good thing to do. Celebrate how long you have had loyal customers.
Good luck.
Mike