(CNN) -- Forget the 4G marketing hype. Which U.S. carriers really offer the fastest mobile-data networks? According to a new report from RootMetrics (a company which conducts its own field tests of wireless networks), Verizon Wireless currently offers the nation's fastest 4G -- by far.





RootMetrics recently compared 4G networks from the four major U.S. carriers in 27 cities. Across all cities, Verizon performed at 4G speeds in 66% of tests.





What qualifies as 4G speed? According to RootMetrics, that's a download speed of at least three megabits per second -- fast enough to download a 30-minute standard-definition TV episode in less than 10 minutes.





"In cities where we could test Verizon's LTE (network), their average download speeds were roughly equivalent with the other carriers' maximum download speeds. In several markets, Verizon's average download speeds actually exceeded the maximum speeds of the other carriers," the report said.





What about the other carriers? According to the report, "AT&T, T-Mobile, and Sprint offered similar performances to one another overall and jockeyed back and forth behind Verizon across the markets. AT&T and T-Mobile were especially close: AT&T offered 4G speed in 40.7% of our tests, while T-Mobile recorded 4G speed in 39.5% of our tests."





Of course, carriers do not deploy 4G networks evenly across a given metro area. They tend to focus deployment in more densely populated areas, and along major highways and office or shopping districts. Depending on the city and the carrier, they may also deploy more 4G resources in wealthier neighborhoods.





So the report noted: "In those cities where LTE was not available, Verizon performed markedly slower. Where LTE is present, Verizon is easily the fastest; where LTE is not, Verizon is among the slowest carriers for data delivery."





Most mobile users don't download huge files to their phones, so "upper 3G" download speeds (1.5-3 Mbps, according to RootMetrics) will still seem pretty impressive to a typical mobile user. And even "lower 3G" speeds (between 500 kilobytes/second and 1.5 Mbps) are adequate for most mobile activities. On this front, AT&T's HSPA+ network is pretty dependable, said RootMetrics:





"AT&T delivered 3G speeds more consistently than the other carriers: they recorded upper 3G speed in 28% of our tests and lower 3G speed in 20% of our tests. AT&T fell below 3G speeds in only 11% of our tests, the lowest percentage of any of the carriers."





T-Mobile's performance was mostly similar to AT&T's in terms of 4G, but "disparities emerge when comparing T-Mobile's non-4G tests to AT&T's. When T-Mobile didn't hit 4G, they ... were more prone to slipping to slower 3G speeds."





Sprint may offer more consumer-friendly 4G data plans than AT&T or Verizon -- but there's a performance tradeoff. RootMetrics found "a significant disparity between cities where Sprint had WiMAX available and those where WiMAX was either not present or access to it proved sporadic. For instance, Sprint delivered a market-best average download speed of 4.6 Mbps in WiMAX-enabled Kansas City, while recording a noticeably slow 0.5 Mbps in non-WiMAX Buffalo."





If you like to post or live-stream audio or video from your phone to the Internet, you probably care about upload speeds -- which means you might want to avoid Sprint, unless it offers a WiMAX network in your city.





"Sprint's average upload speeds were consistently slow. Sprint was the only carrier to never record an average upload speed above 1.0 Mbps in any of the markets we tested," said the report.





Sprint also markets cell phone service through several discount carriers that focus on no-contract mobile plans, such as Virgin Mobile and Boost Mobile -- and some of these brands are starting to market 4G phones and service.





Julia Dey, vice president of marketing for RootMetrics
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compared 4G networks from the four major U.S. carriers in 27 cities. RootMetrics of at least three megabits per second at least three megabits per second "AT&T, T-Mobile, and Sprint
question: what did root metrics do, answer: compared 4G networks from the four major U.S. carriers in 27 cities. | question: WHo compared them, answer: RootMetrics | question: which means 4G speed?, answer: of at least three megabits per second | question: What does $G speed mean, answer: at least three megabits per second | question: Who were Verizon better than, answer: "AT&T, T-Mobile, and Sprint