I found this podcast from this 
post:
I subbed today for a 7th and 8th grade teacher. I’m not exaggerating when I say at least 50% of the students were at a 2nd grade reading level. The students were to spend the class time filling out an “all about me” worksheet, what’s your name, favorite color, favorite food etc. I was asked 20 times today “what is this word?”. Movie. Excited. Trait. “How do I spell race car driver?”
I've only listened to one episode so far, but it's really well produced, seems well-researched and very well put together.
From what I gather so far, the ways that the American public school system "teaches" kids how to read is not only completely wrong, but actually saddles them bad habits which fundamentally hinder their reading comprehension.
A huge swath of American adults are functionally illiterate, and I think I'm starting to understand why.
Honestly, with my personal experience I am surprised it is even that high. I would personally guess more around 70% - 75%, with some rural areas dipping as low as 65% - 70%.