Mountain bike tires vs. road tires - from crushed limestone to pavement I've had my mountain bike for three summers now and I love it.  I bought it for the ability to ride on different terrains and build endurance.  Since my primary goal is weight loss and cardiovascular health, I've found that I ride mostly on paved surfaces during typical rides.  I do often ride on bike trails for long endurance rides.  These trails consistently have crushed limestone surfaces, as well as pavement.

I've considered purchasing road tires due to my extensive presence on pavement, but how well do road tires perform on crushed limestone and/or tightly packed gravel?  Do road tires sink a little on crushed limestone, causing lots of friction?

I don't want to spend the money on the conversion just to find the answer on this question.

I will be riding on paved &amp; crushed limestone/gravel during the same ride.
Gravel tires are normally a little knobby:
 versus completely smooth for a road tire. 

One strategy is to run a gravel or combination tire in the front and a road tire in the back. A combination tire is one which is nearly slick in the middle with knobs on the sides, so you might want to try one road tire on the back and one gravel tire on the front. Combination tires aren't great at road or off road, but they're passable on each. Certainly will be an improvement if you're running big knobby tires on the road, but not as much of an improvement as if you went to complete slicks. Two combo tires might be more durable than a combo/off road tire and a road tire though. 

The main issue is that gravel is sharper than asphalt, so you'll want a bit tougher of a tire. On the other hand, if you're mostly on the road, knobs are bad. So you'd have to play around with it to some extent. 

For what its worth, I currently have an old mountain bike with Geax Evolution tires on the front and back (they're combination tires). If the gravel is packed enough and you're not overly zealous, they seem to work fine. The rolling resistance on the road isn't bad either (though they are a bit noisy, and you'll want to play with tire pressures for gravel and road). Another good option might be something like Kenda Kwick or WTB All Terrain or Specialized Burrough (all slightly more expensive and not exactly equivalent, but the idea is there).