Can I ask how fast is too fast?

I don't really know figures or anything. I was just tossing out a number. 200 sounded nice and round. Plus, what I meant by her being in a tunnel is that the conditions are completely 100% optimal. She'd never find something like that in the field, but theoretically she could be that fast. She probably doesn't even know it- or what the ramifications of that would be to her mind and body.

So... please tell me what a better number is, because I actually don't know what 200 mph pertains to.