AT&T’s public relations team proudly Tweets the fact that a new PC World study of wireless carriers shows a marked improvement in AT&T network performance. According to PC World , their testing methodology (they got help from Novarum ) involved 51,000 separate tests covering 850 square miles of wireless cell coverage in thirteen cities. Their conclusion? AT&T’s network speeds have jumped 84% in eight months, with download speeds now 67% faster than AT&T’s competitors. The study also found that AT&T reliability and performance in their trouble markets of New York and San Francisco has significantly improved: AT&T’s download speeds in New York City were three times faster in our latest tests than in our tests last spring; in San Francisco, the AT&T’s download speeds were 40 percent…