4G LTE speeds in the US are getting faster, but our wireless networks may be choking down unlimited data users, according to a new report from Ookla Speedtest. That makes T-Mobile the fastest mobile carrier in the US when looking at crowdsourced speed tests.

It isn't that simple, though. The report notes that T-Mobile is still at the bottom for overall population coverage. Verizon has great coverage and speeds, but is throttling heavy data users. And AT&T and Sprint provide high speeds less consistently than the other two carriers, the report says.

Ookla's new speed score isn't a sheer measure of megabits downloaded. It incorporates low-end, median and top-end performance for both download and upload speed, to give a bigger picture of overall speed. That's a similar approach to the one we take in our own Fastest Mobile Networks study.

Speed vs. Coverage

Ookla, which is owned by Ziff Davis, PCMag's parent company, didn't just look at speed. By seeing where users were taking tests, it calculated that AT&T and Verizon still have better coverage than Sprint and T-Mobile. While the two bigger carriers cover 98-99 percent of the population, the smaller ones are 93-94 percent. That difference adds up to about 20 million people.

Looking at the difference between urban and rural areas, speed tests taken in metropolitan areas were noticeably faster than tests taken in rural areas. While 23 percent of Verizon's tests and 16.9 percent of AT&T's tests were taken in rural areas, only 6.2 percent of T-Mobile's tests were, the report says.

T-Mobile is moving to close that gap this year through a future partnership with U.S. Cellular and by building out a new 600MHz network on wireless spectrum it just bought, the carrier told us.

Much as we saw in our own results, Ookla found that the best carrier varies from city to city. T-Mobile won 40 of its top 100 cities, Verizon won 35, AT&T 20, and Sprint five.

Some of the results can be a little surprising. T-Mobile took Cleveland and Columbus, Ohio, a state that has historically been a weak area for that carrier. Sprint, meanwhile, took Seattle, which is T-Mobile's home town and headquarters.

Choking on Unlimited

Our Fastest Mobile Networks results this year showed Verizon to have the fastest nationwide network, but Ookla's scores show T-Mobile to be the fastest, something T-Mobile hasn't hesistated to plaster all over the place. This report finally explains why.

Our drive tests used Samsung Galaxy S8 phones with special SIM cards that never got throttled. So our report shows what the mobile networks are capable of, with the latest phone technology.

Ookla's crowd-sourced numbers show the speeds that real users are getting, on their own retail service plans. So if a lot of those users are being throttled, it'll choke down apparent speeds.

In Ookla's tests, the number of results with speeds under 5Mbps shot up after Verizon and AT&T started selling unlimited data plans, even while the networks, as a whole, got faster. Part of the issue may be AT&T's new budget "unlimited choice" plan, which throttles all of its customers to 3Mbps. But Verizon doesn't have such a plan.

"Whether these carriers are deprioritizing customers or customers are flocking to slower, more budget-friendly plans, both AT&T and Verizon are seeing an increase of customers experiencing speeds less than 5Mbps," the report says.

Our drive tests back up this conclusion. If Verizon's network was really under strain, we'd have seen our unthrottled speeds get slower, as we did when the iPhone first smashed through AT&T's network. But Verizon's doing fine with throttling turned off.

This amount of throttling is really putting the lie to the word "unlimited," both on Verizon's and AT&T's plans. Ookla's data makes it look like a significant number of Verizon and AT&T users—we don't know how many, really—are hitting a wall, hard, where their LTE gets slowed to a trickle.

"Verizon Wireless has the fastest service in many of the cities we looked at and comes in second on acceptable speeds, but we suspect their use of deprioritization on Unlimited could be bringing down their overall performance," the report says.