Many of the 100,000 visitors to Australia's biggest car show, Summernats, are ditching the burning rubber and heading to Canberra's sex shops.

This weekend marks the most profitable time of the year for local adult stores, with businesses attributing huge spikes in sales to the four-day car festival.

Manager of Hello Sexy Darcy Jamieson said they had seen a 50 per cent increase in sales and expected the figure to increase in the coming days.

"Compared to a non-Summernats weekend there's been about a ... 50 per cent increase in sales," he said.

"We plan for the Summernats weekend to be quite busy.

"We have everyone working full shifts. We make sure we have enough stock for everything."

Mr Jamieson said despite the rise of internet pornography, X-rated DVDs were the big ticket items this year.

"We get a lot of Summernats people from out of town come here because it's the one Canberra thing to do ... go to sex shops," he said.

"It's the only place in Australia you can buy porn legally so we have a bigger range of X-rated DVDs than other states."

Robbie Swan from the adults only association EROS said Summernats was an "interesting phenomenon" for the adult industry.

"The modern adult industry does not have a lot of connection with fast cars and burning rubber," he said.

"Let's face it there is not a lot of connection on the surface of that but the two just seem to work in this symbiotic relationship."

Sales record still held by church conference: sex shop

Mr Swan expected some stores to double their takings over the four-day period, but said the record for the most profitable day in Canberra's history would remain the domain of a church conference.

"Summernats has traditionally been the most profitable weekend that the industry has," he said.

"The only one that ever rivalled it was when the World Council of Churches held their international conference here back in the late 1990s."

Despite this year's healthy sales Adam and Eve manager Darren Devalentin said there had been a downward trend in sales in recent years.

"There's been a definite shift in the market towards women ... while boys are playing with cars, women are out shopping," he said.

"But in the last five years trade has dropped because Summernats has become more family friendly."

Hotels at full capacity due to Summernats surge

Souped-up Summernats cars parked at the Carotel Motel. ( ABC News: Alkira Reinfrank )

Other sectors of the business community are also feeling the boost, with the Australian Hotels Association reporting all accommodation in the area to be at or above 80 per cent capacity.

"We look forward to it every year, we can accommodate about 730 people in the rooms," Canberra Carotel manager Robert Henry said.

"We're fully booked."

Mr Henry said his motel, which is within walking distance of the Summernats festival, was already filling up with reservations for next year's event.

"We've got guests that [have been] coming for 20 years, and they like to book the room on departure so they hold the room for the next year," he said.

"That happens with the majority."

The hotel is in such high demand that management have employed a security guard to stand watch at the front gate to ensure only Summernats guests come to the motel site.

Dickson businesses see boost from visiting rev-heads

Australia's biggest car festival is running until Sunday and is expected to bring $20 million to the capital.

Wes Faulkner, manager of the Bean & Grain cafe in Dickson, said the area was overrun with people for Summernats.

Extra stock was ordered by Dickson Woolworths in order to help manage demand. ( ABC News: Adrienne Francis )

"You do see an increase in the population around here," he said.

"It's good for all the small businesses here in Dickson ... we see a lot of people coming through in the morning grabbing brekkie, it's good."

Council of Small Business chief executive Peter Strong said Summernats was a much appreciated boost for business, during an otherwise quiet season.

"This time of year, when you're in business you get a bit annoyed that so many Canberrans go down the coast and disappear, so this is a nice replacement."

Even the large supermarkets have been catering to the Summernats guests, with Woolworths in Dickson creating a temporary drive-through checkout with all the essential items sold in bulk outside the store.