I'd avoid [using the UTM projection][1] in order to compare areas across six different zones because of the different areal distortions. Instead the [Lambert azimuthal equal-area projection (LAEA)][2] is the best option, because it preserves area. For instance, you can use the *"US National Atlas Equal Area"* (`EPSG:2163`) or you can define a custom one for Mexico simply changing the coordinates of the natural origin used in the CRS.


  [1]: http://gis.stackexchange.com/a/234077/115
  [2]: https://en.wikipedia.org/wiki/Lambert_azimuthal_equal-area_projection