Try gently pressing your finger against your larynx when you read in your head - you'll likely find you read faster - this works for me, although I tend not to bother.
The worst thing about the inner voice is it's not just when reading, it won't be quiet at bed time. And I have a tendency to complete thoughts as words. Even if I know where it is leading, and what the conclusion is, it feels like I have to see it through and phrase it correctly in my head. I feel very uncomfortable leaving it hanging.