I am also curious why you say I should start with 2-3 psi less (in which is in the ballpark according the formula I found). I am just trying to understand your logic.
No, my logic was to use the different geometry of the contact patch that you would likely see with a wider and or 'longer' tire. (The contact patch is what you would see on the ground if looking from underneath on a piece of glass.)
It would be wider, since you are going from 245-255. It would be marginally
longer, with a larger OD.
If the width is 4% wider, then the area is probably 10% larger (hence the better road feel... try 275s up front!
) so if you have 10% more area, to support the same weight of the car, then you can use 10% less air.
Of course the contact patch can be reduced- if you put the same psi in the new tires, the car will 'ride higher' and the area of the patch will be exactly
the same -in square inches- as it was with the old tire. Remember, if you have 900lbs on each wheel, and you have 30psi in the tires, your patch will be 30 sqin (30x30=900).
So my recommendation actually calculates a new psi that gives you the advantage of the larger tire and gives you a larger contact patch.
But, as with everything on the internet, who knows if this is 'right' or 'exact' or correct. It is a good place to start, and see how your wear, ride and handling progresses.