The retail industry backed into the top spot in Advertising Age’s “100 Leading National Advertisers” list, spending more on measured media than any other category.

As the auto industry cut budgets, retail took over as the No. 1 advertising category, shelling out $18.7 billion. Both categories cut their ad spend, but automakers cut more — down 6.4 percent last year, compared to retail’s 2.1 percent decline.

What else did we learn from this data dive?

Newspapers are still hugely reliant on retail ad revenue (down 2.1 percent last year), which doesn’t strike a person as good news for the news business (down 5.1 percent last year). Retailers spent nearly $6.4 billion in newspapers in 2007, nearly a quarter of total ad spend in that medium. Macy’s spent $609.7 million in newspaper ads in 2007, 43.9 of its ad budget. California stalwart Fry’s Electronics sank 73.5 percent of its ad budget ($229.3 million) into newspaper ads — more than Sears’ $200.7 million, spread across the whole country.

Internet ad spend grew 33 percent last year across the board, but it’s still only 4 percent of megabrands’ ad spend. Retailers did their bit to grow this number, tossing $1.26 billion into the Internet pot.

Who are the big retail spenders on advertising? In order: Sears, Macy’s, Target, J.C. Penney, Home Depot, Wal-Mart, Kohl’s, Lowe’s, Best Buy, Kroger, eBay, Safeway, Circuit City, Gap, and Walgreen. eBay was the only online retailer to make the Top 100 list.

Of the big advertisers, Wal-Mart has the lowest ad cost as a percentage of sales — 0.4 percent — and the highest market share — 10.8 percent.