I find that significant drag only comes when the tyre is under 40psi, that's for a fairly fat tyre >1.5" wide and narrow tyres are much more susceptible to low pressures.thanks for the reply it is appreciated one question would the lower psi slow me down and create drag what do you think, thanks again windycycle.
There will be a maximum pressure the rims can take as well and this is often lower than the markings on the tyre, however I've not known a rim fail and I've got my tyres running at 95psi* when the max should be 50psi for the tyre size. The biggest danger of too much pressure is the inner tube forcing it's way out between the rim and the bead, then you get a blowout. You can get a good idea how likely this is by how difficult it is to fit the tyre, if the tyre almost falls off the rim then don't push the pressure too far.
MPs seem to run badly at lower pressure, more so than normal tyres.