Wireless carriers make all sorts of claims when trying to sell data plans to smartphone users. It can be hard to sort out which carriers advertise their 3G and 4G networks as the fastest, most reliable, and most widespread. To cut through the confusion, PCWorld has tested all of the networks and provided some definitive evidence for which carrier really is the fastest.

PCWorld tested both 3G and 4G speeds for each of the four major U.S. wireless carriers – AT&T, Verizon, Sprint, and T-Mobile. The carriers were asked to send phones that they thought would perform best on their networks. Those phones were used to record data speeds in cities all across the U.S., including Atlanta, Boston, Chicago, Dallas, Denver, Las Vegas, Los Angeles, New Orleans, New York, San Francisco, San Jose, Seattle, and Washington D.C.

The study concluded that AT&T provides the fastest average 4G download speeds in the country, with an average of 9.12 mbps. Verizon comes in second with an average of 7.35 mbps download speed. Verizon does best AT&T when it comes to upload speeds, though, with a 5.86 mbps average vs. AT&T’s 4.91 mbps average upload speed. T-Mobile and Sprint come in at 3rd and 4th, respectively, in both categories, though T-Mobile’s 5.53 mbps average download speed is a lot more competitive than Sprint’s 2.81 mbps. In fact, Sprint came in a distant fourth in every category.

Almost as surprising as Sprint’s abysmal showing is that when it comes to 3G, T-Mobile is tops in download and upload speeds by a landslide. The carrier averages 3G download speeds of 3.84 mbps, well ahead of the runner-up AT&T’s 2.62 mbps, and is even 1 mbps faster that Sprint’s 4G average download speed. Verizon and Sprint aren’t even really in the running with 1.05 mbps and 0.59 mbps download speeds, respectively. This same order holds for 3G upload speeds, though Verizon and AT&T are relatively close. T-Mobile won both 3G categories in all but 2 of the cities where speeds were tested – Dallas and Chicago, where AT&T has slightly higher speeds.

So, if you are looking for a new smartphone data plan, you really can’t go wrong with any carrier besides Sprint. In fact, if you are in an area that doesn’t yet have 4G service from any of the carriers, you might consider the superior 3G speeds of T-Mobile, which has less-expensive data plans than both Verizon and AT&T. Keep in mind, though, that these numbers are a national average. Data speeds can vary depending on your city and where you spend your time. To help with this, PCWorld also split up the data by city.