Tires: Load index, inflation pressure, and oversized tires, 2002 mazda millenia, passenger car tire

Question
I have a 2002 Mazda Millenia sedan that originally came with 215/55R16 91V tires calling for 32psi front / 29psi rear according to the door jamb tire placard.  Interested in increasing both grip and ride comfort, I mounted 225/60R16 98V tires on the factory 16x6.5 rims and so far I am happy with the results.  However, moving from a tire with a load index of 91 to a tire with a load index of 98 seems rather considerable.  I was curious as to some of the math behind the inversely proportional relationship between load index and inflation pressure.  How does the +1 LI -1 PSI ratio hold up with a 7 digit difference in the load index?  I'm currently running 28psi front and 25psi rear or 4psi less for each tire.  I’m thinking that even with such seemingly low pressures, I’m still doing better as far as load capacity per tire than 91LI rated tires at the Mazda recommended inflation pressures.  Do I need more air in my tires?

Answer
Nick,

You have to be very careful here because you might get into the difference between Standard Load (SL) tires and Extra Load (XL) tires, if you confine the discussion strictly to Load Index.

But in this case, both tires are Standard Load, and the rule of thumb is that 1 LI = 1 psi.

But there is a lower limit as the inflation pressure also holds the tire on the rim and applies a frictional force to prevent the tire from spinning on the rim.  I do not recommend use lower than 26 psi on a passenger car tire.

But if you are interested in handling, then why not use the placard pressures?  That's what road racers do!