Good decision to cancel your Mexican papers, as you say you will probably exceed their 6 month validity after taking the time to mozy down through Mexico, then doing GT and BZ.

Yes , the Mexico exit to Belize I was describing is the only big 24 hour road crossing from Quintana Roo state, 20km west of Chetumal .It is at the south end of highway Mex 307 at the town called Subteniente ( "Subte.") Lopez, 4km south of the main highway Mex 186 to Chetumal. . The location on the other side of the border is called Santa Elena Corozal,BZ. This BZ "village" is now turned into a big duty-free shopping zone , access only for foreigners and Belizeans crossing to Mexico.

There are no restrictions on the time between cancelling and getting new tourist cards and import permits to re -enter Mexico. This can be done at Subte.Lopez. There are a few other small border crossings into Guatemala where the Mexican customs personnel are only authorized to check passports and even a few with no official presence at all. If you were to cross at one of those it would ,officially, be necessary to first visit a full-service crossing to do the business , tell them when and wher e you are going to cross and only then could you go. On the GT side you would then have to go to the nearest SAT office to announce your arrival and get "papered".
The crossing east of Tenosique,Tabasco, at El Ceibo has, I believe, now been converted to a full service office with limited hours , and the road into Guatemala has been upgraded into all-weather gravel ( soon to become paved) to El Naranjo,with SAT office . Pavement was laid a few years ago connecting El Naranjo to all points in GT .

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http://advrider.com/index.php?thread....207964/page-5 then scroll down to post #93