By Alina Selyukh

WASHINGTON (Reuters) - Top U.S. automaker General Motors Co (GM.N) has emerged as a heavyweight ally of the wireless carriers battling against stricter Internet traffic rules, with a new letter that linked the issue to wirelessly connected cars.

In a letter to a Federal Communications Commission official viewed by Reuters on Wednesday, GM's Global Connected Consumer executive director, Harry Lightsey III, urged the FCC to "retain the critical distinction" among rules for fixed and mobile Internet traffic and said new limitations for the mobile industry may constrain innovation in connected cars.

"From our point of view," Lightsey said in the letter dated Oct. 9, "mobile broadband being delivered to a car moving at 75 mph down a highway - or for that matter, stuck in a massive spontaneous traffic jam - is a fundamentally different phenomenon from a wired broadband connection to a consumer's home, and merits continued consideration under distinct rules that take this into account."

The FCC is considering whether it should undo exemptions previously applied to mobile carriers as it rewrites "net neutrality" rules, after an appeals court rejected the 2010 version of the rules in January.

Net neutrality refers to the idea that Internet service providers (ISPs) should enable equal access to all Web content.

Under the 2010 rules, all ISPs were banned from blocking users' access to websites, but wireless providers were only banned from blocking applications that competed with their own voice or video calling services.

Wireline ISPs also could not block or "unreasonably discriminate" against applications, while wireless carriers had no specific anti-discrimination rules.

Now, public interest groups and Web companies, including Google Inc (GOOGL.O) and Facebook Inc (FB.O), say the FCC should impose similar anti-discrimination rules for wireless and wireline ISPs. They say that exceptions for "reasonable network management" should be enough to adapt the rules with more flexibility to the wireless networks.

But mobile carriers say compared with wireline providers, they rely on distinct and dynamic technology, carry signal through limited and shared airwaves and face different competition realities that warrant fewer restrictions.

"Any regulations must be governed by flexible policies that are reflective of mobile's extremely competitive market ... and be designed for the unique challenges that our networks face, millisecond by millisecond," Meredith Attwell Baker, chief of the wireless association CTIA, said in a speech on Wednesday.

FCC Chairman Tom Wheeler, at a recent net neutrality roundtable, told a CTIA executive he could not accept a "hell will freeze over" analogy for the possible dire consequences caused by stricter mobile Internet traffic rules.

Instead, he sought to focus the conversation on how wireless carriers might be allowed to "reasonably manage" their networks.

Echoing the CTIA, GM's Lightsey argued that such standards, which are interpreted by the FCC, would create uncertainty for the future.

"The Commission can't define exceptions for 'reasonable network management' for circumstances it can't imagine," Lightsey said.

"By needlessly constraining the latitude our mobile network operator suppliers have in delivering their connectivity to owners of our vehicles, you would also constrain the innovations we are seeking to provide to our consumers."

(Editing by Matthew Lewis)