A name is never given to Tally's city. It is north-east of Diego, and east of The Rusty Ruins. In Bogus To Bubbly, there is a map showing North America in the future. The rusty city nearest to Tally's city is Seattle, Washington.
Diego is assumed to be San Diego, but in the future. Diego does not follow the Committee for Morphological Standards regulations about surgeries, and the fashion trends in Diego are very diverse, but at times bizarre to outsiders. The New Smokeys distributed "The Cure" in Diego, and partnered with the local government. Diego supplies the Smokeys with the cure, sneak suits, a city to take refuge in, and removal of the lesions by a doctor in a safe hospital setting.
The Rusty Ruins
The Rusty Ruins are west of Tally's city and are the remainders of Seattle, Washington. They have been preserved so that children can see what the Rusties did to the world. They are a gathering place for uglies who want to trick the sensors on their hoverboards. David meets up with potential Smokeys in the ruins.
Post Rusty Singapore
As part of the extras plan to launch a shuttle into space so that they have a smaller effect on the earth, they harvest metal from Singapore to build their shuttles. Aya, Hiro, Frizz, and the Cutters visit the ruins with the Extras.
Yokohama is the home city of Aya Fuse in Extras. The cities name is never mentioned in the book, but in early drafts when Hiro was the main character, he refers to the city as Yokohama. The city is not shown to have any rusty ruins near it, so it is presumed to be built over the remains of the modern-day city of Yokohama, since Aya describes her apartment in Shuffle Mansion as having a view of mountains and of a coast; modern day eastern Yokohama has a view of both. Also, the large mountain that Aya and the Sly Girls visit multiple times in the story is presumed to be Mt. Fuji.
Only mentioned once in Extras, when Tally says that metal has been disappearing from the rust ruins near the city. It is presumed that Londinium is built near or on the ruins of London.