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AT&T announced Friday it will offer an unlimited data option for its contract-free GoPhone users,Watch Scott Pilgrim vs. the World Online so even more people have a shot at taking advantage of the unlimited data plan arms race between the major U.S. mobile carriers. Unfortunately for AT&T's prepaid customers, however, the 4G service will be sloooow.

The plan offers GoPhone users "unlimited" data for $60 a month at a max speed of 3Mbps, with 480p video streaming at a max speed of 1.5Mbps. AT&T put the same cap on prepaid monthly data usage as its contract-based unlimited plan, slowing line access after 22GB "during periods of network congestion."

The GoPhone deal is cheaper than AT&T's contract plan by $30 a month -- but customers will need to have auto-pay enabled and the network speed is markedly slower. AT&T's network has an average 4G speed of about 7.93Mbps, according to a study from OpenSignal.

That's a rough proposition for customers who eclipse the 22GB data barrier during a month -- with already slow speeds, the 4G network would likely become borderline painful to use at 128Kbps. It's a cost prepaid users will have to bear to avoid the monthly contracts that go along with the other unlimited plans, though, so they'll have to take what they can get.

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