Is 12 mph a good average speed for a Mountain Bike?

Then I was doing my best to hold the highest sustainable speed over 14 miles. My mountain bike has knobby tires and full suspension (I think, I'm not sure what rear supsension looks like) and the terrain was a fairly flat paved bike trail. I don't have a cyclocomputer, I just time myself then do the math.
Also, is there anything I can do to make my bike faster and how much faster would I go with a road bike?

12 is a decent pace. To make yourself faster, ride more often.

The key to remember is that your exercise is not speed dependent. It is effort dependent.

Your heart and legs have no idea how fast you are flying down the path. They only know how much effort you are expending. Speed only matters in a race.

A road bike would be likely 30% faster, depending on gearing. Over time, your fitness will improve and you will be able to pedal faster too.

12 mph is a reasonable speed for the kind of mountain bike that you've described. You could make it about 20% faster if you got 1-1/2 inch slick tires to significantly reduce your rolling resistance. Make sure that your tires have a max pressure of at least 80 psi. I once cheaped out on a pair of slick tires for an MTB, ending up with a 60 psi max tire. I didn't like it very much.

Don't be tempted to do a spend a lot more on your MTB other than the slick tires to make it faster on the roads. The things you can spend money on are really not cost effective. If you can adjust your shocks, make them as stiff as possible when riding on the road to avoid wasting energy.

MTBs and road bikes are designed for completely different purposes. A real road bike would be at least 30-40% faster than your MTB. The reasons are rolling resistance, aerodynamics, weight, gearing, rider position, and frame geometry.

HTH

12 mph is not too bad on a paved trail with a FS mountain bike and knobbie tires. Get rid of the knobbies and you'll go at least a couple mph faster. Get a light weight road bike and you'll be able to go even another couple miles faster.

Once you get close to 20mph you are spending much more energy moving air out of your way than making the bicycle roll. At this you need a highly efficient aerodynamic position like you acheive on the dropped handlebars of a road bike.

Hope this helps.

Yes, 12 MPH is a good speed for a mountain bike.

Road bikes may be fast, but a fully streamlined recumbent is even faster:

http://www.speed101.com

great pace with a MTB. put some narrower rims and some slick tires on the bike- that will make a huge difference.