We helped Alan to build up his 650b unicycle wheel. This is a new size of tyre that sits between a 26" and the 29" one. His review of the project is enlightening.
*Although only 4% bigger, the increase in wheel size is quite noticeable! Particularly in high gear, you can feel how much further you are going per turn; but it is also noticeably less manoeuverable than my old 26. I'm sure I'll get used to this more, but there were hills I failed to get up (in low gear) that I would have made before.
*Combine the larger diameter with the wheel being substantially lighter, and a fast-rolling tyre, and my cruising speed (137s, high gear) has gone from about 12mph to about 14mph. That's quite an increase...
*The 650B Velocity Blunt 35 rim saves about 200g over my previous Halo Combat 26" rim, and the new Spirit cranks are substantially lighter than my old Moments. Tyre weight is pretty similar - I went from a Halo Twin Rail 26*2.2" to a Schwalbe Hans Dampf 2.35" (standard rather than supergravity version), both weigh about 850g.
*The tyre is quite a change from my previous Twin Rail; it's really knobbly, but for such a knobbly tyre, it rolls extremely quickly on tarmac! And off-road grip is of course great, although its mud-shedding isn't in the same league as, say, the Kenda Karma. Cornering (particularly on tarmac) was also horrendous at first, I was all over the place, but am gradually getting used to this.
*The Spirits also have a 110mm-crank-length hole; I was only able to give this a brief try (on tarmac) but what with the lighter wheel, this was surprisingly manageable (in high gear, so that's a ~~41" wheel) - more so than I remember my 26" as being with 125s! But didn't really get a chance to test this on our mostly-offroad route.
The ideal would be to build a 26" uni that went as fast as this, but had the smaller wheel's manoeuverability; and TBH I suspect a lot of the "improvement" is due to the weight saving, so that might even be possible. I also have a Schwalbe Racing Ralph tyre (27.5" * 2.2", 550g) waiting to try, which could be interesting!