Wheel won the account after a four-way pitch against undisclosed agencies. The incumbent was PHD.
Around 60 per cent of easycar.com's marketing spend will be used online.
A company spokesman declined to give a budget figure, but the site has spent between £2 million and £5 million a year on marketing in recent years.
Wheel's media director, Caroline McGuckian, said the agency won with a proposal showing how it could generate 70 per cent of bookings via online marketing.
Wheel's initial focus will be on targeting outbound hires - UK tourists looking to hire a car overseas. The next stage of marketing, which the agency has planned for the first quarter of next year, will target inbound travellers.