Greg Fountain, who has been editor of the 108,200-circulation monthly for three years, said his departure had been planned for some time.
'There are big, exciting plans for the magazine that will become apparent. We are expanding, and it is positive all round,' Fountain said.
Car is looking for more journalists ahead of the changes later this year, he added: 'My first priority is to work with the publisher to find my successor.'
He estimates it will be September before his replacement takes over and one person in the frame for the job is deputy editor Matthew Franey.
'He will be very much part of the transition plan,' said Fountain, who intends to write a book in his free-time. He will continue to act as a consultant to Emap.