How do on-road mountain bike speeds translate to road bike speeds?
On my reasonably flat commute I average around 16 mph on my road bike, but your average speed is dependent on many different factors. A general rule of thumb is that if you are switching from a mountain bike with knobbies to a road bike you will be between 15-20% faster at the same watts/effort.  Typically that's only a change of 2-3 mph.

I teach a bike commuting workshop, and one of the most common questions is whether to switch from a mountain bike to a road bike in order to increase speed.  Typically, I tell people to try three things first:


Swap out your knobby tires for high pressure slicks.  You can find 1.25" -1.5" slick tires that fit mountain bike rims and run at between 75 - 90 lbs of pressure. These will dramatically reduce rolling resistance.
If you have a suspension either lock it out, or set it as stiff as you can.  Locking out your suspension will cause more of your effort to be transferred directly through the drive-train and translate into less loss of momentum from shock absorption.
Try clipless pedals.  Your pedal stroke will be more efficient, again resulting in an increase in speed.


If you do those three things the only real differences between a road bike and your mountain bike will be the weight (which matters a lot more when accelerating than it does when you are already rolling) and surface area resistance from riding in the drops.  But most road bikers don't spend much time in the drops.

Finally, your weight and fitness make a huge difference.  I frequently pass road bike riders while on my commuter rigid frame mountain bike with high pressure slicks.