An even simpler way would be to include a marker to denote the end of the shell script, and the start of the data. For example, if you put this in extract.sh

    #!/bin/sh
    sed -E '1,/^START-OF-TAR-DATA$/d' "$0" | tar xvzf -
    exit
    START-OF-TAR-DATA

and then run:

    cat extract.sh ../foobar.tar.gz > foobar.tar.gz.sh

You can then run foobar.tar.gz.sh to self-extract. And you still get the benefit of being able to modify the shell script without needing to count lines or characters without sacrificing any compression.