I cannot remember a reference, but I have seen results of an actual test of the change of effective rolling radius with tyre pressure. The result is that for radial tyres, there is no appreciable change in rolling radius for any realistic change in pressure.

The reason for this is obvious if you think about it. The distance travelled for one revolution of a wheel is equal to the length of tread laid down on the road during that revolution. For a radial tyre (even for a crossply pretty much) the belt prevents any stretching of the tread with changes in pressure, until the pressure gets so low that the belt buckles, carrying the load on the sidewalls and sliding the tread. This means that the same length of tread is laid down in one turn of the wheel, regardless of the pressure, even if the length of the contact footprint increases at low pressures. For it to be any different implies that either the tread and hence the belt has changed in length with pressure, or the tread is sliding on the ground.

So I conclude that changing the pressure to equalise the rolling radius for different size tyres simply will not work.

Anyone who wants to check this can do so - simply find a hard, level surface, put a chalk mark on a tyre, and on the road, move exactly one (or more) revolutions of the wheel, mark the road and measure the distance between the marks. Repeat for a different pressure. The distance will be the same within the accuracy of your work for any practical pressures.

John

__________________
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol