Andy’s comments are correct, except where he suggests that the wider tires are needed for high torque bikes. Actually the reverse is true. The issue here is the width of the rim relative to the tire. Just levering on a wider tire doesn't result in a larger contact patch. A wider tire on the same old rim just pushes the tire out at the center resulting in a more rounded profile. A smaller tire, such as a 170 on a rim designed for a 180 tire results in a flatter profile and increased contact patch. Buy like Andy says, going up on the tire size, let alone two sizes is a sure way to mess up your handling on aggravate a few other things as well. Even the resulting overall width looks pretty impressive until you put it on the skinny rim and resulting increase in width is very slight, usually around 3 or 4 mm. The only reason the 'too wide a tire' results in any increase at all is that it angles away from a proper sealing position against the rim lip.