T-Mobile used information from speed-testing sites like Ookla and Open Signal to back up its claims. Verizon argued that because it began offering unlimited plans (which slows down customers' data when it goes past a certain monthly amount) during this period, many of its users would have seen slower speeds near the end of the month. The NAD, part of the Advertising Self-Regulatory Council (ASRC), agreed that the situation favored T-Mobile unfairly, and recommended that T-Mobile stop claiming that it has the fastest speeds.

The self-regulatory body also reviewed other T-Mobile claims, including one that it covers 99 percent of the geographic area as Verizon does. The NAD found that T-mobile cannot support this claim, as it only covers the same amount of people as Verizon. The NAD recommended that T-Mobile modify its advertising to make this clearer. The changes to advertising claims are likely only temporary. T-Mobile told Ars Technica that while it would comply with the NAD's recommendation for now, it reserves the right to say that it has the fastest LTE network based on future data.

Update: T-Mobile has clarified the NAD announcement on its ads and claims of being the "fastest" network. According to the carrier, the ad in question was pulled because it referenced Verizon's lack of an unlimited data plan. Verizon has since re-introduced that option, so the ads were removed. T-Mobile also says that the NAD recommendation was based on old stats and stands by its claim of being the fastest carrier with new numbers. The company did not offer those figures in its response to Engadget.

Here's the full statement from Janice V. Kapner, SVP of Corporate Communications at T-Mobile: