Janet Schürmeyer and Magdalena Schaarwächter started taking pictures of their male friends in front of polished-up cars of a certain age in 2009.
"I was doing my diploma in design and was making a car magazine aimed at women, which is when I came up with the idea," Schürmeyer told The Local.
She and Schaarwächter, a photographer, worked together to make their first calendar, consciously making fun of the traditional calendars featuring bikini-wearing women lounging across high-powered cars.
"We only use male models - actually most are our friends - and they are often not wearing the most attractive underwear," said Schürmeyer, speaking from her office in Wuppertal.
The cars they use are also not Ferraris or Maseratis but include a Citroen 2CV, an AMC Pacer and a Fiat 500. "There is a minimum age of 20 years for the cars. For the men it is 18," she said.
They called their calendar Autowäsche, a combination of the German words for car and underwear. The subtitle reads, "Milestones of men's underwear meets automobile history."
Having started with a print run of 400 for the 2010 calendar, the pair have expanded to produce around 2,500 for 2013, as well as a range of table mats bearing a selection of cars and men in their pants.
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Buyers are not only women, said Schürmeyer. "I would say it is about equal with men and women buying them. It is not a question of gender; rather it is a question of a person's sense of humour."
But she said she and Schaarwächter were not planning a calendar featuring women. "I don't know if it would work with women. There are only a few calendars with men and cars and we have a niche now."