These days, the U.S. carrier conversation appears to dominated by Verizon and T-Mobile — for good reason.

America's networks are all getting faster, according to a report by Ookla-owned Speedtest released this week.

But T-Mobile is the fastest network, and Verizon, despite prevailing in large cities like San Diego and New York City, is nipping at its heels. AT&T is slightly behind, and beleaguered Sprint brings up the rear, despite making large strides in the last year.

According to Speedtest, T-Mobile's rise to the top has been spurred by a number of factors, including the acquisition and deployment of additional low-band spectrum, refarming of existing spectrum that was previously used for 3G service, and the proliferation of "capacity enablers" like 4x4 MIMO, 256QAM, unlicensed LTE, and a focus on selling efficient Gigabit-enabled phones like the Galaxy S8 and upcoming LG V30. With average speeds of 23.17Mbps, T-Mobile isn't that far ahead of Verizon at 21.13Mbps and AT&T at 20.05Mbps, but it's enough to maintain CEO John Legere a stockpile of verbal ammunition when talking about his biggest rivals.

Average mobile download speed in the U.S. increased 19.2% between Q1-Q2 2016 and Q1-Q2 2017 to 22.69 Mbps. That is not as strong as the year-over-year growth of 33% we saw in last year's report. The U.S. still only ranked 44th in the world for download speed, immediately behind Fiji and Germany and just ahead of Oman for Q1-Q2 2017.

Verizon's capacity advantage does help in urban areas, though: the company's second-place narrows considerably against T-Mobile in big cities where spectrum is at a premium and millions of people try to connect to overloaded towers. Verizon still does it this better than T-Mobile, for the most part, and had fewer signal drops. On the other hand, Verizon may not own the speed crown in rural areas — that's T-Mobile — but it completely dominates in terms of coverage, according to Speedtest.

Although the majority of each carrier's tests do take place in urban areas, some carriers have a much larger footprint in rural areas than others. The coverage maps above give a sense of this, but the numbers are even more telling. Verizon accounted for a full 51.6% of all samples we saw in rural areas. AT&T made up 27.3%, T-Mobile 11.5% and Sprint 9.6%. Verizon's rural coverage is laudable.

Speedtest does not a drop in speeds from Verizon and AT&T since the introduction of unlimited plans in February, something that CEO John Legere and co. continue to use as justification for switching to T-Mobile.

Our data shows that in the case of Verizon and AT&T, the percentage of test results with the lowest-end download speeds (those under 5 Mbps) shot up compared to the period before these unlimited data plans were widely available. For comparison, both T-Mobile and Sprint are seeing the opposite effect in the same time period where fewer results are below 5 Mbps in Q2 2017 than they were in Q4 2016.

In other words, T-Mobile and Sprint, due to fewer people on the network, have been better able to weather the storm of increased load on the networks than much-larger Verizon and AT&T.

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Of course, speed is just one metric; the differences between top speed and acceptable performance are enormous, and both T-Mobile and Verizon are able to offer the most consistent experience, or in Speedtest's parlance, an "acceptable speed ratio", compared to AT&T and Sprint.

The metrics are fascinating, but all this comes back to a point that we've been stressing a lot in the past few months: AT&T, despite maintaining its considerable install base lead over T-Mobile, is no longer at the top of the conversation when it comes to U.S. carriers. It's now T-Mobile vs. Verizon in most peoples' minds.

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