Notes on buying tyres,
various way of measuring tyres dating back as much as 100 years or so has left a confusing legacy.
for eg. there are 5 or 6 differnt sizes of 26" tyre and both 28 and 29 (-er) inch go on smaller wheels than 27".
the ISO way of measuring tyres is the best and the one to be relied upon, it's expressed as two numbers.
for eg. 40-559 this is a tyre of 40mm width and 559mm BSD (bead seat diameter - which is the diameter of the inside of the tyre where you can feel a wire running through)
although there are ideals of rim width to tyre width, in practice the width of a tyre is very forgiving, for eg. the same wheel that takes a 28mm width tyre will probably hold a 50mm width tyre
BUT it is this 2nd number (the bead seat diameter) which is critical to the tyre you need to fit your wheel. Look for that number on the wheel or on the tyre already fitted to that wheel.
as well as expressing tyres in some of the older terminologies on my listing (for eg. a 622 tyre can often be called a 28" a 29" or a 700c)- I always give the ISO number.
When you are looking for a tyre, please determine the bead seat diameter of both your wheel and make sure it matches the tyre you intend to purchase.
559 (26" mountain bike) or 622 (700c/28"/29-er) will be by far the most common for modern adult bikes)