I'd avoid using the UTM projectionusing the UTM projection in order to compare areas across six different zones because of the different areal distortions. Instead the Lambert azimuthal equal-area projection (LAEA) 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.