Sitting in a £24bn-a-year market, Motors is the second largest used car site in the UK behind Autotrader. Tesco's takeover of Carsite.co.uk and its launch of Tescocars.co.uk earlier this year have added to competition in the market.
Motors.co.uk commercial director Phil Jones said: 'We recognised that we needed to take an integrate approach to our comms strategy to make the biggest impact.'
Diffusion is the first retained consumer PR agency for the website, which previously used PR support for trade media only. The agency will combine traditional PR with a social media campaign.