Not sure why that calculator doesn't offer more tire selections. The MTB I have has 26x2.60 tires but the cal only goes to 1.75 commonly found on Walmart type of bikes. Plus is the cal using knobby tires like mine on the mtb selection? they don't say. Just wondering what the difference would be between going with a 32 pound mtb bike with knobby 26x2.60 vs 19 road pound bike with 700x25's? But it is a very interesting caluculator.