Originally Posted by
krobinson103
I have both. An mtb slicks with a good top gear ratio is only marginally slower than a roadbike. The roadbike wins with a head wind and riding over 45kmh but I don't do that all that often so it bothers me not.
You can get very good rolling tires for 26" wheels. Assuming you don't have suspension on your MTB, then the largest difference between it and a road bike is aerodynamics, and you'll notice that effect most at 15 mph or larger. (And you can make yourself more aerodynamic on a MTB, too).
So, small speed difference (unless you're really riding quickly) and as for fitness, no difference.