Burning DVD: -speed

[sam]

I have some problem burning dvds, i use this command line (burning data
making iso on the fly)
growisofs -speed=X -Z /dev/hdc -iso-level 4 -J -r /path/to/data/

but the X speed (1-2-4-8-16) is simply ignored, and the drive choose
itself the speed (and every time could be different)
i really need to choose the burning speed, because i have reading
problems in many device with my current burning options.