Automotive ad dollars rate among the biggest revenue sources on the Internet. The industry spends so much that it was partially blamed for Yahoo’s woes when one financial quarter saw lowers sales for display ads by the auto makers.

But the money continues to rev up the Internet, with eMarketer projecting a rise in online auto ad spending through 2012. At that time, they expect car makers and related advertisers to fork over more than $5.6 billion in ad revenue.

The spending reflects a simple trend. Car buyers use the Internet to research their purchases, and everyone connected to the industry wants to get in front of those potential customers.

eMarketer cited a shift in trust from family and friends providing word of mouth recommendations, to those opinions expressed in online customer reviews.

Seeking opinions about vehicles topped the list of reasons why people go online for car information. Capgemini in 2007 cited 51 percent of people planning to lease or buy a new car within the next 18 months as those looking for thoughts about their prospective purchases.

One does not suffer from a dearth of opinion online about automobiles. Blogs like Autoblog and Jalopnik feed the demand for details, as do countless media outlets and message boards devoted to the car.