The speed selector gives you the available burn speeds for the inserted DVD-R disc. If you havent insert any disc it gives only up to 2.4x. This doesnt mean anything.
Each DVD-R disc has a maximum burn speed. Look at the discs before you buy them. Also look at sony site for recommended and supported DVD-R media.
If you get a max burn speed 2.4 x with an inserted disc then this disc should be +RW. 2.4x is not an option for -R/-RW.