Just use your original tires until they wear out and then replace them ..Personally I use my mountain bike on pavement and off road so I prefer to run knobby off road tires all the time. There are some knobby tires out there that roll pretty fast on pavement when pumped up hard....Running narrow slicks on a mountain bike is just wrong, it's like putting low profile rims and racing tire s on a Jeep, looks terrible.