My Arita 48x (Ritek) CD's burn at 48x-
... and make perfect coasters
At 16x or less, they're readable at least.
The speed is set by the firmware and hardware, but depends on the disc quality. Sometimes the 3500 decides to choose a lower speed, because it can detect that burning at a higher speed will result in an unreadable disc, a bit like AOPC on DVD writes. I noticed that with one of my other CD brands, it is rated 24x but the NEC doesn't allow any higher than 16x.
Btw, I noticed the same behaviour with my LiteOn 30123@40125s, I could burn unrated or 8x rated CD's flawless at 40x while some rated 48x discs weren't allowed any higher speeds than 24x or 16x.
Generally spoken, if the drive doesn't allow a higher speed, you can be pretty sure that the disc would turn out to be a coaster anyway.