Located southeast of Mexico City, Oaxaca area has several areas for mountain biking. A good starting point for local singletrack is in San Antonio Huitepec. Check out the radical vertical descent into San Fernando de Matamoros rocks.
Fly directly into Puerto Escondido from Mexico City via AeroMexico ($75).
Note: Prior to 1997, the topographic maps developed by the federal government used the NAD27 CONUS datum. Starting in 1998, the government transitioned to the ITRF92 datum (some maps show both datum), which is pretty much identical to the WGS84 datum.
Overall, San Antonio Huitepec is the 6th most popular mountain bike trail of all 24 mountain biking rides in Mexico.