How does tyre pressure affect rolling resistance?

[Mussels]

I'm thinking of getting new tyres, and I would like to get the most suitable ones. It seems that a semi slick is the appropriate tread and that is almost what I have, thin tyres are out as I ride over some sand and gravel.
My current tyres are inflated to a max of 60 psi, if I were to buy slightly narrower tyres (1.75 instead of 1.9) that inflate to 80 psi then would I notice a difference in the rolling resistance?
I ride almost 40 miles a day every day and mainly on road so it could make a big difference, the answer seems obvious to me but I know experience doesn't work like that so has anyone here made a similar switch?

 

[flecc]

The two cancel out, a thinner tyre at a higher pressure having the same rolling resistance as the wider one at lower pressure, as long as the pressure change is proportional and all other factors are equal. In the example you give, I doubt you'd notice any difference, though the thinner tyre could feel less comfortable, especially on an unsprung bike.

I changed to thinner and better tyres on my T bike, but the difference in rolling resistance is not noticed much unless I raise the pressures disproportionately.

Very high pressures as used on race bikes markedly decreases the rolling resistance of course.
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[Mussels]

The two cancel out, a thinner tyre at a higher pressure having the same rolling resistance as the wider one at lower pressure, as long as the pressure change is proportional and all other factors are equal.

That sounds to me like a fat tyre at low pressure is about the same as a thin tyre at high pressure, are you sure a mountain bike has the same rolling resistance as a road bike?
So if I'm looking for new tyres I should look for the same width but higher pressure?

 

[flecc]

The treads have to be equal, as that's one of the "all other things equal", and mountain bikes rarely have free rolling treads, so in the main they roll badly.

Same width higher pressure will reduce the rolling resistance, but very often the best approach is to have the tyre with the lowest rolling resistance.

Schwalbe have a good reputation for low rolling resistance tyres and they have an excellent PDF of their technical information including a whole section on rolling resistance that you'll find useful, well worth downloading:

Schwalbe TechInfo
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[Mussels]

Very informative, I was suprised to see how much more important wind resistance is even at low speeds.
Perhaps I need to start shaving my legs. Hmmm