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.

Knobby tires are the biggest difference. 3 mph difference (where 15 mph on road and 12 mph in mountain bike) is what I'd expect.

Weight doesn't affect too much on level ground, but does make hill climbing slower. Mountain bikes are also usually geared lower than road bikes.

If you want to use a mountain bike on the roads and you don't go off road, then putting non-knobby tires makes a huge and immediate difference.

Cheers,
Charles

Knobby tires, tire width, and a less aerodynamic riding position.