There is a bit of confusion regarding Sonora vehicle permits, and the fact that Rocky Point/ Puerto Penasco (in the State of Sonora) is a permit-free zone.
So… which is it? Sonora Only, or permit-free?
When you travel in Mexico, all of Baja and the area west of Mexico Highway 15 to the coast of the border with Baja is permit-free; also the area from Douglas-Agua Prieta via Mexico Highway 2 west through Cananea to Imuris and everything north of that highway to the US-Mexico border. The southern end of the free zone is at KM98 on Mexico 15, just south of Empalme. Communities in the Free Zone include:
Here are some maps of Sonora:
Vehicle Permit Sonora (TIP)
There is NO REQUIREMENT for Sonora Tourist Permit in San Carlos. The trip from the United States requires that foreigners obtain a tourist visa (FMM), but they are not a car permit if they are traveling by private automobile. The requirement is only if you travel south of Guaymas. The area where these car permits are available is referred to as “El 21” as it is 21 kilometers from the border or at the border itself.
The key is Highway 15. East of 15 in Sonora, get a Sonora-only permit (cheaper when you buy it at the Banjercito, but not when you buy online). So, there’s Baja and the Free Zone (half of Sonora – described above), and then there’s the other half of Sonora that qualifies you for a cheaper special permit, and finally the rest of Mexico.
You may not need a Tourist Permit in Sonora, but you still need the usual paperwork: driver’s license, proof of car ownership, passport, mexican car insurance, etc.