If you’re American or Canadian, you have free entry on a tourist visa for six months, but you need to request that at the airport or if you drive across. It is automatically granted to everyone.
Under the tourist visa, there is no restriction on what you can do other than you can’t work, but you still need to do the process.
If, as an American, you just drive across the border and show the officials in Mexico your passport, they will not just give you a six-month tourist visa. You need to request the visa, and based on the judgment of the official that is there, they will give you a certain amount of time you can be in the country under that visa. They can give you 1 month, 2 months, or up to 6 months but that depends on the custom agent. You could request more time as well.
As an example, if you’re driving a brand new, very luxurious van, you are very well dressed and you speak very nicely and you’re very kind, they will give you the 6 months, but if you’re crossing in a terrible car that seems very shabby and dirty, they will give you less by far. They have their own criteria.
If you want to be in Mexico for more than six months, go out of the country, come back again, and they will give you more time. That’s the rule.
If you have a dog with you, just make sure you have the proper permit.
If you have a legal car, it is not a problem to bring it to Mexico. If it’s legal in the US, it is legal in Mexico. They will give you the tag when you’re crossing.
(Border crossing at Tecate, California into Tecate, Mexico near San Diego, California, pictured.)