My three-month-old bike (Huffy Rangeline 27.5), the back tire is worn smooth around the middle. It's a mountain bike, but it has street tires. I rotated them, so in another three months the back tire (which was the front) will probably be worn smooth as well.

I ride 14 miles a day. I purchased two $20 Wal-Mart knobby tires for my old Trek and they lasted a lot longer than three months.

Is that normal wear?

So, when I finally need tires what kind should I buy? Knobby or street? Which brand? What price ranges am I looking at? Where should I get them?

Any suggestions appreciated