Just got a pressure gauge which claims to be accurate to within 0.5 psi. Bontrager Mud-X, 2.0F 1.8R – 17 psi front, 21 psi rear. 12 stone on a Boardman HT Pro. Riding the clay and chalk of the South Downs and woodland singletrack. Shocked at how low they are – when I pumped them up a couple of months ago the track pump gauge had them at 28 and 34 psi, so not sure how much is pump gauge inaccuracy and how much is deflation over time. Rolling well off-road and tons of grip.

Usually run Speed Kings and Bontrager ACX's at 40psi. Did a race though a couple of weeks ago and had a tubeless blow-out owing to a cut – however managed to stuff just enough air in to keep going once it sealed. I didn't bother about the pressure, just straight back on as soon as it stopped hissing and had a tiny bit of pressure (was on last lap). I was astonished at the extra grip and it still rolled well, as well as not being flung in the air all over the roots – really made me think I should drop my pressures a fair bit. I checked it at home on the track pumnp and it read somewhere a bit below 20psi.

Never really thought to experiment with pressures, have only just gone to tubeless and still in the pinch flat mindset.

http://www.michelinbicycletire.com/michelinbicycle/index.cfm?event=airpressuremtb.view