I travel by car to Mexico a lot, when you get 20-30 miles south of the border, and you are a gringo. Be prepared to get stopped and have your vehicle and documents checked often. A few palm greasing bucks should be in your wallet, but hide the bulk of any travelers checks or cash.
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Since March 1, 2010, all U.S. citizens – including children – have been required to present a valid passport or passport card for travel into Mexico. While documents are not routinely checked along the land border, Mexican authorities at immigration checkpoints approximately 20 to 30 kilometers from the border with the U.S. will often conduct vehicle and document inspections and will require valid travel documents and an entry permit or Forma Migratoria Multiple (FMM). All U.S. citizens entering by land and traveling farther than 20 kilometers into Mexico should stop at an immigration checkpoint to obtain an FMM, even if not explicitly directed to do so by Mexican officials. Beyond the 20-30 kilometer border zone, all non-Mexican citizens must have valid immigration documents (FMM, FM2, FM3)
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maybe someday they will loosen up their rules for non-citizens as they expect the US to do. It's a great place to travel through.

Mexico does require non-citizens to abide by rather stringent rules and plenty of 'gringos' have experienced having to pay a bribe or two after showing their documents. I think Mexico is the pot to Arizona's kettle.