When i was ordering my parts up for the Surly Carbon Krampgrease build i needed some tubes as i have never used this 29+" tyre size before. The Surly ones were in stock so ordered a couple as they were the right size for the tyre/rim combination. Then i thought what is the biggest normal 29" inner tube on the market - quick search and Continential MTB 29 S42mm are the biggest stating they go upto 2.5" so 1/2" short of the Knard 3". What the hell i ordered a couple to give them a go and see if the would stretch or go bang! At first i built the wheels up with the Surly tube - no problems then i tried the Conti. Straight away out of the box its about 1/2 the size/thickness - no way, this is going to go bang. I fired up the compressor and took it steady. It went up ok and didnt go bang and the valve looked ok. Well what the hell i decided to give it ago and get some miles in to see what would happen. I can now report back after 100's of miles use they are no problem. You have got to over inflate them to repair punctures and probably like the michy ones i have in my 26+" wheels chuck them after a few repairs. They are holding strong with only one area of tyre misshaped, adjacent to the valve area so the tube must not stretch enough at that point, you dont feel it when riding. The rrp on the Surly £12 430g, Conti version if you shop around £3.05 235g so that is a weight saving of 390g for a pair and nearly £18 saving in your back pocket as well !
Again with more manufactures taking out / designing 29+" tyres hopefully they will take out inner tubes to match and get some competition going to reduce the rrp's