Polk Co., a Southfield, Mich., automotive marketing information firm, yesterday completed its purchase of Carfax Inc., a Fairfax company that sells data on the history of used vehicles to consumers and car dealers.

Polk, in a partnership with Blackburn Group Inc. of London, Ontario, already owned 35 percent of Carfax and now has acquired the remainder. Both Polk and Carfax are privately held, and no purchase price was announced.

Carfax provides detailed information on the history of vehicles to consumers and more than 9,000 dealers in the United States and Canada. The information includes a car's state titling records and whether it has been sold at a salvage auction, had its odometer turned back or been damaged in a flood. Carfax said about 70 percent of the sales of its reports are to dealers who are contemplating buying a car at auction or taking ownership as part of a trade-in.

Individual customers looking for a used car can purchase an unlimited number of such reports over a two-month period for $19.95. Carfax said it recently set a single-week company record of more than 9,200 sales to individuals.

Polk, among other ventures, collects data on vehicle sales trends, which it sells to car dealers and others interested in tracking the buying public's ever-changing tastes. It said it plans to operate Carfax, with its 120 employees, as a wholly owned subsidiary and keep its current name. Polk said Dick Raines will continue as Carfax president.

Stephen Polk, chairman and chief executive of the Michigan firm, said in a statement that it bought the remaining two-thirds share of Carfax in part because of "the excellent Internet delivery system techniques Carfax has developed for selling directly to consumers" at its World Wide Web site (www.carfax.com).

Raines said he welcomed the Polk takeover, saying it would give Carfax "access to advanced data-processing capabilities" and "a richer variety of data."