ImgBurn shows you the speeds supported by your media.

When you insert a blanck CD, and ImgBurn is in Write mode, look at the right hand window of ImgBurn.

Mine shows:

Disc Information:
Status: Empty
Erasable: No
Free Sectors: 359,847
Free Space: 736,966,656 bytes
Free Time: 79:59:72 (MM:SS:FF)
Supported Write Speeds: 16x, 24x, 40x, 48x

So I would never use 1x, 2x, 3x or 4x for my CDs.