I find most spoken audio very comfortable at double speed. Whether this matches reading speed depends on your reading speed. I also find spoken audio less fatiguing than reading for most fiction and most popular nonfiction. I am less prone to being derailed from the book. This is where I read for a bit and then find something else more compelling, such as a blog or a rationalist forum, or where I pick up the book and then put it back down having resolved to continue tomorrow. Even if reading-time is shorter when I read, calendar time is often very long.
Anything I could comprehend at double speed would probably not be worth listening to.
Audio may take longer, but perhaps that enables me to remember it better. Any studies on that?
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http://itunes.apple.com/us/podcast/harry-potter-methods-rationality/id431784580