With several major new mobile broadband services launched -- or coming soon -- in the U.S., and all promising blazing performance, it can be hard to get a clear picture of what wireless speeds you will actually get when you plug in your new data card or switch on that minty fresh new handheld or netbook.

The official word from AT&T Inc. (NYSE: T), Clearwire LLC (Nasdaq: CLWR), and Verizon Wireless is that their new networks will be faster than the maximum speeds offered today. But not landline-replacement kind of fast.

In fact, the fastest-sounding new network is also the one with the least known about it. Indeed, Verizon doesn’t want to talk numbers at all on its Long Term Evolution (LTE) network until it has done a lot more testing outside the lab. But we can look to other users' experiences, particularly in the case of AT&T’s planned High-Speed Packet Access (HSPA) upgrade, to glean some indication of how these networks will operate in the real-world.

AT&T
AT&T has just unveiled its timetable to update its 3G network with a technology called HSPA 7.2, work that is expected to start this year and be completed by 2011. A spokeswoman for AT&T says that -- as you might expect -- the network upgrade will offer a “theoretical maximum speed of 7.2 Mbit/s” compared to the 3.2 Mbit/s peak rate currently available.

The operator says the new update "will primarily affect downlink speeds." It notes that uplink speeds will be between 500 kbit/s and 1.2 Mbit/s.

We can, however, look to Vodafone Group plc (NYSE: VOD)’s deployment of the 7.2-Mbit/s upgrade in the U.K. to get some general idea of how AT&T’s network might perform. Heavy Reading analyst Gabriel Brown tested the network in June last year and recorded some performance figures.

Brown found that in central London locations where 7.2-Mbit/s service was enabled, service was consistently in the range of 3 to 4 Mbit/s data throughput on the downlink while stationary, with bursts as high as 5.5 Mbit/s. Stationary uplink offered a throughput of around 1 Mbit/s. 3G in motion was not quite so blazing, however: “300 kbit/s is what you can expect on a 3G connection while on the move across London by car or train,” Brown wrote at the time. (See Vodafone's Blazin' 3G Upgrade.)

There's no guarantee that AT&T’s network will perform exactly the same, since each market topology is different and can affect performance, amongst many other variables. Still, the Vodafone experience does offer a starting point for user expectations.

Clearwire
Clearwire is a little more forthcoming about the average speeds of its new Clear mobile WiMax networks. "Average download speeds range from 2 to 6 Mbit/s," a spokesman tells Unstrung. "Upload speeds are typically up to 1 Mbit/s." The company says the network can deliver burst peak rates of "more than 10 Mbit/s," but there's no telling how close you have to live to the cell-tower for that to happen.

Clearwire’s WiMax service is currently available in Atlanta and Portland, Ore. When Clear’s precursor, the Sprint Corp. (NYSE: S) XOHM service launched in Baltimore, Unstrung saw download speeds between 1 Mbit/s and 3.5 Mbit/s in the various tests around the city, although the Clear networks are said to be somewhat different than the original Sprint offering. (See XOHM's Big WiMax Demo Day.)

Verizon
Verizon is the most tight-lipped of the three about the speeds that will be offered by its LTE network, which is due to start going commercial in 2010. The operator has only said that it has seen between 50- and 60-Mbit/s downloads with the technology in initial tests. A spokesman for the company says that it doesn’t want to set customer expectations until it has done "a lot more testing outside of the lab" with the proto-4G technology.

"The bottom line is that until you load the network you can’t know for certain -- you can make an educated guess, but you can’t know," the spokesman says.

Unstrung has very little hard data on how LTE runs, since the first commercial base station just went live in Sweden. (See Sweden Claims LTE First.) Motorola Inc. (NYSE: MOT) did, however, stream video at 8 Mbit/s on an LTE test network at the CTIA show this year. This may give some indication of the raw speeds but doesn’t yet help to account for how the network will perform when lots of users are roaming on it.

— Dan Jones, Site Editor, Unstrung