T-Mobile USA has upgraded its HSPA+ network in five new markets and claimed that unlocked iPhone 4S devices running on its 1900MHz network record download speeds that are an average of 70 percent faster than on AT&T’s network.

In a blog from CTO Neville Ray, T-Mobile said the network has been upgraded in Chicago, Reno (Nevada), Fresno and Sacramento (both California) and Southern California, bringing the total number of metro areas to benefit from the enhanced network to 23. “These upgrades  will now provide voice and data enhancements, improved signal strength and  in-building coverage for customers,” noted Ray.

Ray said customers in the covered metro areas can switch their unlocked AT&T devices to T-Mobile “and experience a significant speed boost on our network, while also saving up to $50/month compared to AT&T.”

Ray claimed the total number of people who can now access the upgraded network stands at 100 million, with similar enhancements being made in Boston, Los Angeles, New York, Philadelphia and San Diego.

T-Mobile is making the most of its HSPA network – both in terms of technical upgrades and marketing publicity – before moving to LTE next year. Its main rivals have already launched commercial LTE networks.