Nielsen Media Research figures reveal it accounted for 13.5% of all ad spend on the medium during the 12 months to the end of February 2008.
According to the new data, presented by Enders Analysis, motoring advertisers' share of radio spend during the period was up by 2%, compared with the same timeframe a year earlier. The sector spent a combined total of £79.9m on radio during the period.
The second-biggest spender on radio was "government institutions", such as Whitehall departments, which spent £79.8m during the 12 months to the end of February, a 10.8% increase year on year.
Meanwhile, entertainment and media advertisers were the third-biggest spenders on radio, investing £75.8m. Among all categories, cosmetics and toiletries brands posted the biggest expansion, increasing ad spend to £13.2m during the 12 months to the end of February 2008 from £8.8m a year earlier.