The conflicting ads are everywhere. AT&T (T) claims to have “the nation’s fastest mobile broadband network.” Verizon (VZ) Wireless says it has the “best network.” And Sprint Nextel (S) brags that its early rollout of “4G” technology gives it “industry-leading mobile data services.”

In fact, there’s not a lot of real-world difference between the three, according data in a new report from Akamai (AKAM).  An Akamai survey based on a huge number of wireless downloads toted up the speeds of the big three U.S. carriers and found barely a dime’s worth of difference in the three networks’ average and peak download speeds.

Mobile download speeds in the third quarter of 2010 averaged 1 megabit per second on the big three U.S. mobile networks.  (While Akamai doesn’t say which network is which, it hardly matters, since the differences were so small.  The precise average speeds of the three networks, in megabits per second, were  .97, 1.06 and 1.08.)  Similarly, peak download speeds were also tightly clustered, averaging 3.5 megabits per second, with a low of 3.3. and a high of 3.7).

This does not mean that AT&T customers’ complaints about poor wireless phone service, such as this recent Jon Stewart rant, are necessarily unfounded.  Part of the way all networks have managed to boost data speeds is by limiting the capacity of their older voice networks. The Akamai tests say nothing about the dropped calls that Stewart parodied.

The new report, which also described the continued growth and speeding-up of the wired Internet, is based on a unique look at the health of the worldwide network.  As the globe’s largest “content distribution network” Akamai increasingly sends web pages, songs and videos to the world’s mobile networks. The company’s servers then record the speed of those downloads, giving it an unusual and independent view of the changing wireless world.