My Mountain bikes are efficient- so are my road bikes. Same motor so not a great deal of variance there unless you want to say so.

I will not ride my Road bikes offroad because they do not work in that situation. I can change the gearing- tyres and put on stronger wheels but they will not be efficient.

I do ride my MTB's on the road but that is only to get to the offroad. Lower gearing- heavier overral weight- suspension that saps energy and smaller wheels with chunky tyres mean that they are hard work on the road- in comarison to a road bike.

Before I went road- I used to do 100milers on the road (And Offroad) on the MTB's. Lower the bars- fit narrower slick tyres and possibly change the cassette to give me closer ratios gears. Never had a problem. These bikes worked on the road. Then I got a road bike at about 1/5th the value of my MTB and sorry---- Mtb's do not work on the road.