Does the manual have a multi-element table showing recommended pressures for each size combo ? I not, you can play some games by measuring the differential pressures needed to save wear & tear. I recommend that you take into account whether you will weight the tires before you start this. I also suspect you are asking about pressures while in 4wd.
1st: set the rear pressures between the sidewall amount somewhere the ride is acceptable. Then bleed down gradually the fronts and back the tractor up on sand. When the best pressure is reached, there will be perfect impressions of two superimposed tread blocks. That implies that the rolling distances are the same. I do it in stages of constant pressure instead of a bleed rate.
Next: Run the tractor up to a moderate speed and shift into Neutral when passing a landmark. Let the machine coast down to fully stopped and mark the distance traveled. This test essentially tracks the rolling resistance of the 4 tires. At some set pressure, the extra rolling resistance coming from the mismatched tires will shorten the distance coasted.
Yes, temperature, wear, FEL load, rear weight changes all affect this, but once in a while it would make sense to repeat the process.
BTW your machine will appreciate the matched pressures when it's time to disengage 4WD because the conflicting drive shaft loads can bind up a clutch and gear sets.
Run this as a science project by some school kids and they'll get an award for Best in Class !
