I have* to cycle to work tomorrow so in preperation I've swapped the wheels on my do-it-all bike from the ones with slicks to the ones with tread. The knobblies are 2" and the slicks are 1.5" (wheels are 26" btw) and it always annoys me that the knobblies have a larger circumference. It means that the mudguards are too close - the wheels turn and everything, but it's more snug than I'd like. In an ideal world there would be more clearance with the knobblies on to account for snow, mud and all that - but in real life there is too much of a gap with the slicks, and not really enough with the knobblies. Am I right in thinking that if I had both tyre in the same width, they would also be the same outer diameter? Or is it not that simple? I've looked on my tyres for a marking giving the sidewall height as with car tyres (is it expressed as a percentage of width on car tyres?) but there doesn't seem to be anything. Ideally I'd like narrower slicks and to keep the 2" knobblies - is there any chance I can get tyres that will have the same outer diameter? Or do I need to go to the bike shop with a ruler? * or walk. But you can forget that.