For all intents and purposes, when considering a 2.25 tire... 650B is 27.5" 26" mtb is not 26"...it's 26.5. That is why it is confusing. I have been researching MTB wheel sizes for a little over a year now and I've learned quite a bit. When I first looked into the 650B MTB size I assumed it was in the middle of 26 and 29. I soon realized this was incorrect. If you look at the pics that Jacquers posted, and run the numbers you will figure it out. The discovery is not that 650B isn't 27.5"...because it is. In fact, a 650B wheel with a 2.25 tire is closest to 27.5" than either of the other wheels are to their namesake sizes. A 650B wheel and 2.25 tire is 27.496" so you may as well round it up to 27.50". Take a look at the 29'er wheel with a 2.25 tire, it's 28.976" or rounded up 28.98"...pretty close to 29" anyways. The thing nobody here has mentioned though is, look at the "26 inch" MTB wheel and tire. It is 26.496" or 26.5. So the reason that 650B is not half way in between the 26 and 29 wheel sizes is not that 650B isn't 27.5, it is, the reason 650B is not in the middle is because 26 is really 26.5. Look at the measurements in the diagrams and convert mm to inchs (2.54 cm per inch). Each diagram shows a rim with a tire of about 2.24 inches. Using a 2.25 inch tire makes sense since a 2.25 is exactly the average of the range of tires that many XC and ALL MOUNTAIN chose from, between 2.1 to 2.4. Measure your own 26, 650B or 29 wheels/tires and you'll see. Over at MTBR there are many discussions about wheel sizes, with lot's of talk about 26, 650B and 29. There are so many reasons why people like one wheel size over another, but let's just say some people like the lightweight manouverability of the smaller 26 mtb wheels and bikes and others like the rollover and traction of the 29. Some people argue that 650B doesn't offer enough improvement over 26 to make it worth while. I have yet to test the wheel sizes extensively. One reason why 650B became an mtb size is that bike designers were having trouble making long travel 29'ers with good geometry. 650B was thought to provide a bit more rollover advantage and traction without the problems of frame design. When I first started researching wheel sizes and discovered all the 650 wheel sizes (650 = 597 mm > 650A = 590 mm > 650B = 584 mm > 650C = 571 mm (and remember when GT invented the 700D wheel size for their hybrids? 587 mm), like another poster here, I thought 650A would have been the proper middle size at 590 mm. It is exactly in the middle of 559 mm 26" wheel and the 622 mm 29'er. That could have mde sense, but eventually, I believe the ideal wheel size could have actually been the 597 mm wheel size. It is exactly 1 inch smaller than 29'er. Perhaps that is all that would have been needed to make a smaller wheel that was another option that worked. I personally think that this wheel size should have been tested. Since it's only 1/2" bigger than 650B, thoeretically, I bet a 597 mm wheel with a 2.25 tire could fit on a 650B bike since it's not much bigger. Many 650B wheels fit on 26" bikes even though the wheels were 1 inch bigger. The difference between 584 and 597 mm is only 1 inch. It may seem like splitting hairs, but I still think it could be a workable option. And the argument that a 26" wheel with a large tire is the same as a 650B is not comparing apples to apples. A 26" wheel with a tire big enough to be 27.5" is going to be alot different in ride and feel than a 650B with a 2.25 tire. One thing that you want to avoid, especially for those trying conversions, where you fit 650B wheels on a 26" mtb, is to reduce your tire size from the one you rode on your 26" bike. If you rode 2.25 on a 26" bike, you're best not to go down to a 2.0 just to make it fit. You lose out on cush and minimize the increase in rollover and traction you would have with 2.25. Anyways, we've got 650B now, so I think it's worth a try. Next year I plan to compare all three wheel sizes. Arguing which is better is not necessary. Some wheels suit different terrain better than others and some people have more fun on a given wheel size. I don't believe any of the three wheel sizes of 26, 650B and 29 are bad. Anyhow, just thought I'd add my two cents from the research I have done. Have fun riding whatever you have.
