I think it's many factors. For example, when I got my first hybrid bike, I did 12mph. After replacing the cushioned seatpost, I did 13-14. Evidently it sapped some peddle energy. After I switched from a large comfort seat to a narrow "racing" seat, I was 14-15. Then clips to hold my feet in the peddles so I could pull up instead of just resting my foot on the peddle... 15-16.

Next I bought a road bike and do 22 easily. Just due to a more efficient riding position, probably lower profile position with less air resistance.

My point is: it's not just gear inches. Going back to my first hybrid, I rode middle chainring and the #7 gear in the back. By the time I was up to 15-16mph, I was on the tallest chainring and #7 in the back (starting to wish I had a higher gear).

So, it's a chicken/egg thing. Yes, gearing is essential to going faster. But, tweaks in riding position, equipment, etc. will be what determines which gear is normal for you.