Mountain bike, one road bike tire. Front or back? I have a road bike tire and was going to put it on my mountain bike since I use it to commute to work. 

Should I replace the front or back tire? Which will give me the better performance?
For efficiency I would put the road tire on the back, as the rear carries more weight. However, front would be best if cornering traction was important. 

If the MTB tires have an aggressive MTB tread pattern the mismatch would be best addressed by matching tires and getting another road tire.