Hey Anthony, I found out what the license plate numbers stand for.
George Carlin does the voice for Fillmore. The numbers on the plate correspond with George Carlin's birthday.
Here's some more stuff that was posted on the Full Moon Bus Club's website:
Fillmore from Pixar's movie Cars is a 1960 VW bus and Radiator Springs' resident hippy. Fillmore is voiced by George Carlin and has over 60 bumpers stickers:
“Recycle Your Oil”
Three “Flo’s V-8 Café” stickers
“Free Mother Road”
“Groovy and Love
“Portland Concrete on My Mind”
“Please Recycle”
“No Interstate”
“Go Natural, Drink Organic Fuels”
“Certified Organic Oil”
“Home Sweet Road”
“I’m an Eco-Bus”
Six “Peace” signs
“Stop the Superslab”
“Your Mother Road Loves You”
A flower
A heart with 66 in it
Oval RS
“Ornament Valley”
“Open Road U.”
A rainbow
“Peace on the Open Road”
A rainbow butterfly
Recycling emblem
A Route 66 shield
“Turn On to Recycling”
“Life Begins at the Offramp”
“My Inner Car is Electric”
A sticker that has a 66 shield on it with a tire track over that says “Tread on Me”
“Recycling: It’s a Gas”
“Save Mother Road”
“Flower Power”
“I (heart) Recycled Oil”
“Drink Organic Oil”
“Free Mother Road”
“Question Interstate”
Recycling symbol with “Take a Drive on the Wild Side”
“100% Organic Fool”
“Fillmore Organic Fuels”
Recycling emblem
“Pardon My Backfire, I Eat Veggie Fuels”
“Save 2-D Animation”
“One Way” (recycling symbol)
“Interstate” inside a stop sign
“I Brake for Jackalopes”
“Support Organic Refiners”
“I (heart) 2 Recycle”
“Recycle Your Oil” with a yin-yang symbol
An oil derrick with a slash through it
“It’s Not Nice to Fool Mother Road”
“100% Organic”
A bigger 66 shield and a heart
“Got Solar?”
“Oil” with a recycling symbol in the “O”
“Born to Recycle”
“I (heart) Recycled Oil”
“Certified Organic Oil”
“Go Natural – Drink Organic Fuels”
A sun with a recycling symbol in the center
A flower
“Drink Organic Oil” with flower in it
“100 Percent Organic Oil”
“Interstate” with a slash through it
66 shield with a rainbow
51237 is the license plate, which is George Carlin’s date of birth
Oh, and I haven't found any, but my brother has one for me down in Florida.