I have a Trek 1000 with 700 x 25 tires. I also mainly ride my mountain bikes on unpaved gravel and dirt roads. I would hate to ride my 1000 on any 300 mile limestone trail. My only suggestion is to get the widest tires you can find for suspension. Don't worry about going knobby. I run semi-slicks on my mountain bikes and they work as good as anything in gravel.