Across the U.S., Verizon and T-Mobile are practically neck and neck in terms of mobile network speeds and performance, according to the latest findings from OpenSignal's crowdsourced monitoring.

OpenSignal also found that 4G speeds continue to rise, with typical speeds in many cities surpassing the national average. Of 36 cities the company studied from October through December, 25 now have one or more mobile service providers that deliver average LTE speeds faster than 20 Mbps (megabits per second).

The city with the fastest 4G speeds in the new study is Detroit, where both Verizon and T-Mobile average LTE download speeds of more than 25 Mbps. Minneapolis and Chicago also have at least one operator with average 4G speeds of 25 Mbps or greater. OpenSignal's results are based on some 4.6 billion crowdsourced datapoints measured through its mobile app, which is being used by nearly 170,000 people across the U.S.

Verizon Retains Top 4G Availability -- for Now

Compared to the results in its previous "State of Mobile Networks" report, released in August, OpenSignal found that Verizon had gained some ground over T-Mobile in its 4G speed rankings late last year.

"Verizon has clearly taken exception to T-Mobile's recent attempts to steal the network spotlight," the new report noted. "Verizon is fighting back with 4G upgrades of its own, and as our latest results in the U.S. show it's doing so quite effectively. The two are again tied in network speeds in our tests, but T-Mobile continues to chip away at Verizon's vaunted lead in 4G availability."

Tests showed that Verizon currently offers the highest 4G availability rates, with those speeds available 88.2 percent of the time. T-Mobile, however, is only two percentage points behind, "the closest we've seen that difference," OpenSignal said.

T-Mobile: 'We're Pushing Industry'

Overall, all four of the nation's top wireless carriers, Verizon, T-Mobile, AT&T and Sprint, realized "significant improvements in their 4G availability scores" over the past six months, according to OpenSignal. While Sprint remains in fourth place in terms of availability, it has improved its performance by nearly seven percentage points during that time, the OpenSignal report noted.

The latest report also finds that a growing number of cities across the U.S. have access to faster 4G speeds: 25 now have access to LTE speeds of 20 Mbps or better, compared to the 18 identified in August's report.

"Verizon recently began upgrading its networks with LTE-Advanced technology, which definitely explains some of the big jumps in Verizon's speed we’ve seen in the last six months in cities like New York and Orlando," OpenSignal analyst Kevin Fitchard noted in a blog post yesterday. "But we see speeds improvements from other operators as well as the industry goes through the methodical process of tweaking LTE performance across the country. For instance, AT&T and T-Mobile have been deploying LTE in new frequency bands and in many cases are using old spectrum they no longer need for 2G or 3G services."

In a statement released yesterday, T-Mobile CTO Neville Ray said the new OpenSignal report shows that his company's LTE network-building efforts have "pushed the industry to try to catch up."