A little help here please :)

Hi, I’ve just bought a LTR-40125S installed no problem, but what I want to do is burn using some old cdrw just to see how fast I can burn them over what speed they are meant to be burnt at, but when I use Nero it won’t let me change the speed even with the smart burn box unchecked, how can I change the speed, I don’t care about errors at the moment, I just want to try burning at a faster speed then what the cd is made for.

Thanks in advance

You can NOT write faster as the speed written on the cd-rw tells you - i.e. SMART-BURN on or off doesn´t matter for cd-rw writing speed!
2x CD-RW won´t be rewritable at 10x, neither would 4x … you´ve to use 10x - 12x for maximum rewritable speed, and 12x is maximum!

To clarify, the LiteOn can only write 12x to a 4x-10x rated CDRW. The older 1x-4x rated CDRW discs can only be written at 4x max.
Nero is telling you the available speeds for the disc you have inserted.