Not sure about your mileage, many factors play into it.
For me, I can get 45-50 for maintained hwy riding, ie. riding longer than 20 minutes.
Anything less than that, like my daily commuting to work, I get only about 35-40 at best.
As far as your knees go, if you don't have the "low seat" option, you could check into a different seat, or you have have to have a custom seat made that sets you back a little bit more.
I have 2 Russel seats, and one gives me a touch more leg room at the front, the other, not so much but the wife likes her perch better.
Something else that can play into your knee room, riding pants. Any tail bone padding and knee armor can have the potential to cut down on your knee space too....