That’s the conclusion Wired.com’s Gadget Lab draws from a survey of 4,200 iPhone 3G owners. The results, posted Monday morning, show marked regional differences that Wired.com believes are primarily due to the maturity — or lack thereof — of the local 3G network, and not some underlying problem in Apple’s (AAPL) hardware.

The study, which invited users to measure their local 3G speeds and enter the data on an interactive map, was less than scientific. The participants were self-selected and a third of them provided data so incomplete the information was unusable.

But the 2,636 data points that were usable — and which Wired.com plotted on a Google map of the world — offer a window into the nature of the iPhone bandwidth problem that has drawn so many complaints.

The most striking differences in 3G reception emerged when the study compared the United States with Europe. Users in Germany and the Netherlands reported the world’s fastest average 3G download speeds — about 2,000 Kbps. The most “0” results — indicating no 3G signal whatsoever — came from users in the United States.

Other results from the survey:

• European T-Mobile (DT) users reported the fastest 3G download speeds: 1,822 Kbps on average. [Wired notes that Europe has some of the most mature 3G networks, which have been in development since 2001. AT&T (T), by contrast, introduced its 3G network in the United States in 2004.]
• Canadian carriers Rogers (RCI) and Fido tied for second fastest with an average download speed of about 1,330 Kbps on average.
• U.S. carrier AT&T tied for third with Telstra (Australia), Telia (Sweden) and Softbank (Japan), where users reported average download speeds of roughly 990 Kbps.
• Australian carriers Optus and Virgin users reported the slowest speeds of about 390 Kbps on average.

The survey also shows striking differences from one neighborhood to another. Manhattan’s Rockefeller Center and upper Fifth Ave. got very strong signals, for example, while coverage in the rest of the city was spotty. [Reception in my Brooklyn neighborhood was reported to be particularly weak, which jibes with first-hand experience.]

One explanation for the poor bandwidth in metropolitan areas like New York and San Francisco is offered by Dave Nowicki, developer of the femtocell, a technology that extends the reach of wireless networks. He suggests that although these cities are richly supplied with 3G antennas, they are also the places where the most iPhone users reside, resulting in overloaded networks and pokey bandwidth.

This is a problem, Wired.com concludes, that won’t be easy for Apple to fix:

In our view, this data is a strong indicator that performance of the mobile carrier’s network is affecting the iPhone 3G more than the handset itself. This also furthers our thesis that it’s highly unlikely that Apple is going to wave a magical wand and say, “3G problems, be gone,” with a software update. Before Apple can make such a claim, it needs to wait for all of its carriers to optimize 3G network behavior — in terms of number of towers, how they’re positioned and how much bandwidth each tower can handle. (link)

To see the results from your city or town, click here and zoom in until the blue dots resolve into bar graphs.