the type of tire will make some difference. but not a whole lot, although i do recommend a wider street tire for more grip in corners. if you have inclines running a 36t might be a little boggy. i would recommend a shift kit from SBPhttp://www.sickbikeparts.com/catalog...js078i50chv8u4
this will allow you to use your rear derailer and gears. if you do like the shift kit idea keep in mind when buying a bicycle, you do not need one with front gears, so a 7 speed bike will be just the same as a 21 speed bike.