The city of Tucson is over 200 square miles, so what’s this “23” all about?

If you’ve driven downtown lately you’ve probably seen this purple sign: “Welcome to the best 23 miles of Mexican food in the U.S.”

This is a pretty heavy claim — and it turns out to be a joint effort by the tourism board, the city and a private tour operator to promote Tucson’s food scene.

The 23 square miles refers to a specific swath of land, including many of what City Councilwoman Regina Romero calls Tucson’s “cultural and culinary corridors.”

Romero worked with Christopher DeSimone of Gray Line tours to pick out more than 50 “generational” Mexican restaurants that represent local culture and history.

DeSimone’s guided tour focuses on the cultural history of many south-side restaurants on the list, like the traditional Sonoran restaurant El Merendero on South 12th Avenue. (You can sign up for his tour at grayline.com.)

The city’s effort is all to promote Tucson’s recent designation as a UNESCO City of Gastronomy, the only one in the United States.