AT&T on Friday promised a "truly unlimited plan" for its prepaid phone customers, but the new offering limits video quality to standard definition, caps overall speeds at 3Mbps, and prohibits mobile hotspot use.

For $60 a month, GoPhone (AT&T's prepaid brand) now offers a plan without traditional data caps and overage charges. “We’re excited to offer a truly unlimited plan for our AT&T GoPhone customers,” AT&T VP Bob Bickerstaff said in the announcement. But the plan is always limited to an overall speed of 3Mbps, and video is limited to 1.5Mbps (about 480p), AT&T said. The cost is $65 a month for customers who don't sign up for automatic payments.

The plan is basically identical to one AT&T recently introduced for postpaid customers. But while this is the only unlimited option for AT&T prepaid customers, AT&T's postpaid customers have the option of paying $90 a month for an unlimited plan without the 3Mbps speed limit. The $90 postpaid plan also includes 10GB of high-speed tethering data a month and lets customers enable HD video.

Network speeds vary by location, but AT&T has an average 4G download speed of 13.86Mbps, according to crowdsourced data collected by OpenSignal last year.

Like other AT&T unlimited plans, GoPhone unlimited customers who use 22GB a month can be throttled during periods of network congestion for the rest of the month. If you're not in a congested area, you keep receiving speeds of up to 3Mbps. This is similar to other carriers, although T-Mobile USA sets its throttle point at 30GB.

AT&T prepaid customers can also opt for a 6GB-per-month plan that doesn't limit speeds to 3Mbps. Video is limited to 1.5Mbps by default on the 6GB plan, but customers can enable HD video, which uses more data. There are no overage fees after you hit 6GB in the month, but data speeds are slowed to 128kbps. The 6GB plan costs $40 a month, or $45 without autopay.