T-Mobile has a marketing problem.

While the carrier was once content to advertise itself as the “nation’s fastest 3G network” with its upgrade to HSPA+ technology earlier this year, it might as well be advertising the world’s largest cassette tape collection. It seems 4G has become the buzzword of 2010, and while nothing about T-Mobile’s HSPA+ networks fits the technical definition of a 4G technology, T-Mobile threw reference books to the wind on Wednesday by claiming it has the largest 4G network in the nation. Needless to say, the semantic stretch has lead competitors and pedantic nerds alike to wring their hands over the technical faux pas.

But forget the scuffle over names. Is there anything to T-Mobile’s claim to speed competitive with existing 4G networks? Armed with T-Mobile’s latest HSPA+ modem, a 4G WiMax-equipped laptop and an iPhone on AT&T 3G for reference, we hit the streets in Portland, which has access to all three services, to see how HSPA+ truly measures up where it counts: in real life.

In the office

Digital Trends’ tenth-floor vantage point in the US Bancorp tower lends itself to great views, but not necessarily great signal. The altitude and structural elements, including copious steel, sometimes interfere with cell reception. In fact, heading toward the core of the building can sometimes drop signal to near nothing. We tested near a large window for the best possible speeds, where WiMax reported three out of five bars, HSPA+ delivered half-strength, and our iPhone appeared to have all five bars.

WiMax hit the fastest speeds in the tower with up to 5.18 mbps, and an impressive average of 4.0mbps. Although it fluctuated as low as 3.23mbps, it still managed to outdo HSPA+ in every single test, which returned no higher than 2.84mbps and 2.57mbps on average. They came close, however, on upload speeds, with WiMax returning 0.65mbps and HSPA+ returning 0.56mbps — neck and neck results that repeated themselves in nearly every test. Our 3G iPhone predictably lagged behind on download speeds, managing only about 0.54mps, but blew both next-gen options out of the water with an average 1.24mbps upload.

On the streets

After taking the elevator down 10 floors and walking out into a sunny fall day, things took a turn for the strange. Despite WiMax reporting stronger signal (four bars) than in our office, speeds dropped dramatically, while HSPA+, reporting the same signal, had speeds shoot up. T-Mobile was able to deliver a consistent 3.5mbps, while WiMax suddenly lost its gusto and averaged only 1.14mbps. Upload speeds on both remained exactly the same. The iPhone didn’t seem to care for our street corner of choice at all, delivering the slowest speed of our test at just 0.16mbps, despite still reporting full signal.

Down below

The first time we tested Clear’s WiMax service in Portland, the parking garage test managed to surprise us. While AT&T’s 3G service grabbed a few bars, WiMax blanked on signal entirely, leaving us totally disconnected. The same happened this time, and HSPA+ also fell victim. While our Rocket modem reported two bars of signal, it refused to connect, leaving incomplete results yet again. The iPhone absolutely crawled (we couldn’t get any faster than 0.07mbps on downloads), but having connectivity at all in a concrete dungeon remained quite impressive. The obvious conclusion: Stay away from both HSPA+ and WiMax if you live in a cave.

On the go

As we did during our first 4G trials, we hopped on the MAX, Portland’s above-ground light rail system, for a more dynamic test of download speeds. Besides simulating how quickly you might, say, load directions to a destination while frantically driving to said location, it moved us through a range of coverage areas to ensure the speeds we were seeing at static test spots weren’t flukes.

WiMax hit the magic five bars as soon as we boarded and delivered an amazing 9.45mbps. Speeds did drop off a bit from that point as we got underway, but it still managed an average of 8.2mbps, and finally broke 1mbps for peak upload speeds. Despite decent signal, HSPA+ didn’t seem to enjoy the ride quite as much, delivering only 2.47mbps on average. At one point, we hit full bars for the only time in all our testing… only to get 1.59mbps downloads. The iPhone, meanwhile, continued to putter along at no faster than 0.42mbps, for an average of 0.2mbps.

Three-and-a-half G

There’s no question that T-Mobile’s HSPA+ drives traditional 3G service into the ground, so we can hardly blame T-Mobile for trying to slink out from under the 3G umbrella and slap on the shiny 4G sticker. But this turbocharged version of 3G still lags behind existing 4G service when both run at wide open throttle. We managed to wring 9.45mbps out of WiMax at its fastest, while HSPA+ never even managed to break 4mbps. You can’t ignore the speed gap there.

But keep your chins up, T-Mobile customers. The relative flakiness of WiMax does shine through in the averages. For all our testing, WiMax averaged 4.29mbps, while HSPA+ hit a relatively close 2.80mbps — a good indicator that they’re not as far apart as the flashy maximum speeds would suggest in most day-to-day operation.

We’ll leave it to the pundits to decide whether T-Mobile is misleading customers by branding HSPA+ as 4G, but one thing’s for sure: It beats the living hell out of vanilla 3G.