How much more effort does it take to ride a common mountain bike at the same speed as a common road bike ?

I know there are many variables to consider but assume the bikes are both aluminum with a weight difference of about 5-8 lbs between them. The mountain bike has standard common knobby tires and the road bike has standard smooth road tires.

This may be totally wrong but it seems to me that for MY same amount of effort on a flat surface I am about 3 mph slower on my mtb.