i have a 2014 specialized hardrock sport 29er & i am looking to buy slick tyres for it. the specialized site says the rim is 29" with 25mm inner width. i have no idea what size tyre to buy? i know i need a 700 but... 23c, 25c, 28c... i dont know what this means & if any of them will be too wide or too narrow for my rims? can anyone help pls?
Like you say 29er rims have the same diameter as road 700c. The difference is MTB are often wider and need wider tyres. 25mm inner diameter is quite wide and won't play with the skinny tyres you mentioned (My skinny 29er rims are about 19mm inner, 23c means 23mm wide). one website I found states "The ideal relationship between tyre and rim section is about 1.8 times, but any tyre from 1.4 to 2.2 times should fit". It has a table suggesting 44 mm minimum. http://www.betterbybicycle.com/2014/04/a-simple-guide-on-essentials-of-wheels.html
You might want to check out cyclocross tyres.
Don't forget to get skinny tubes and to reset the bike computer for the reduced tyre circumference.
I run 700 x 38 slicks , 700 x 42 semi slicks on my rockhopper 29er, anything smaller may have seating issues. May need to get new tubes to suit the new tyre size.