Summernats boss Andy Lopez takes US ambassador Caroline Kennedy on a tour of Canberra car festival

The biggest one-day crowd in the history of the event - 45,000 people - turned out as US ambassador Caroline Kennedy made a surprise, incognito visit.

For the first time - following on from a huge visitor turnout over the first two days - event organisers took the difficult decision on Friday evening to close online and gate tickets for Saturday.

"I think the venue itself could certainly handle more people inside but if we had kept people coming in the concern was for the effect that would have on what is happening outside," Mr Lopez said.

"We were aware of the knock-on pressure on traffic and parking and people getting in and out so we called it."

Ms Kennedy, with a small entourage in tow, arrived at the gates dressed informally in in blue jeans and a long-sleeved black T-shirt and quickly swapped her plain cap for a Summernats one as she headed off around the packed, noisy, bustling venue.

Summernats co-owner Andy Lopez took her on a tour of the main judging pavilion where she stopped to chat with chief judge Owen Webb, chatted with patrons and just like so many of the car fans, took out her phone to snap some photos of some of the elite 60 cars on display.

Mr Lopez said he received the surprise call from a senior ACT government official on Friday asking if he would show the US ambassador around.

"I asked them what she might want to see and they said she wanted to see as much as she could: the elite cars, Skid Row, the cruise strip, all that stuff," he said.

"I said I'd be delighted to do so."

Alex Smith of Canberra was one of the locals with whom Ms Kennedy stopped to have a chat, and he quickly grabbed a couple of mates for a photo with her, still blissfully unaware of her identity.

"That's Caroline Kennedy? Wow, I had no idea; how cool is that," he said when he found out later.

"She asked me if I had a car here on show and I said not yet, but come back next year and you might see it," he said.

Such was the demand that all the black Summernats 35 T-shirts, featuring the late Chic Henry on the front and priced at $50 a pop, were sold out by early afternoon and sellers were even cleaned out of last year's discounted apparel.

All the drift rides for 16 cars were fully booked and the food and frozen cocktail vendors did a roaring trade in the warm weather.

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Hundreds of cars, in almost every variety from T-buckets to rare and expensive Walkinshaw-fettled Holdens, rumbled along the cruise strip as the crowds crammed up to four and five deep along the concrete barriers to watch the passing parade.

Out in the main area, two mates who live only nine doors from each other in Chisholm, Mitchell Paterson and Jay Saleta, took out the fashions in the field prize in their matching pink banana outfits and pink-tufted mullets, pipping Ian Johnson, from Newcastle, who stripped down to his Bundaberg bathing costume.

There were more mullets than ever before, out and proud, with dozens of entries to the competition which included prizes for best "grubby", vintage (over 50 years), "game changer" and "super majestic".

The body art was won by Mark Farnell, from Brisbane, who bared his rear end to show his complete, full colour Japanese bodysuit that took over 300 hours "and a lot of pain" to complete.

Police had been called to Uriarra and Fyshwick on late Friday afternoon to reports of illegal street burnouts. One car was seized. Only 15 traffic infringement notices were issued.

Summernats co-owner Andy Lopez said that given the size of the crowd, behaviour had generally been very good and he was hopeful that would continue.

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