specify compression level in tar gzip

from http://superuser.com/questions/305128/how-to-specify-level-of-compression-when-using-tar-zcvf

GZIP=-9 tar cvzf file.tar.gz /path/to/directory

or

tar cvf - /path/to/file0 /path/to/file1 | gzip -9 - > files.tar.gz