Thanks for the replies. Just to provide context, I am planning to commute about 14km one-way to work. It will be flat on a dedicated bike path (not shared with cars). Traffic lights are minimal, the city designed it to avoid those things by either going over or under major intersections.
Out of the 14km, about 2km will be paved gravel. I am using my MTB with pumped up knobby tires. Hoping to maintain 25km/h.... is that possible?