Tires: How to found the right tire size?, maximum allowable weight, load tables

Question
What is the formula to calculate the appropriate tire size (front & rear) for a car? Using total weight, corner weight, weight balance F/R, horsepowers, center of gravity...?
can this formula be used for all vehicules (small automobile, sportcars, trucks...)?

Answer
Jean-Pierre,

This is a question that needs to be asked of a vehicle engineer, particularly one involved with suspension system design.  Unfortunately, I am not one of those - I am a tire engineer.

But I can give you a little insight into what things are considered.  

First, tires are sized both dimensionally and by their load carrying capacity.  So you have to have access to the load tables as published by the tire standardizing organizations.  These standards vary a bit and a good vehicle engineer will consider what effect all the possibilities have.  However, since these standardizing organizations don't have a product (except these load tables), they don't allow folks to publish them - especially on the internet - You have to buy their books, which are about $60 each.

The second thing that has to be considered is the load on the tire.  Each corner of the vehicle is different so this has to be calculated for each corner.  

Then a "factor" is applied to get what the required load carrying capacity of the tire should be.  What this "factor" is has never been revealed to me.  I can only tell you that there is no agreement among vehicle manufacturers and that over time it has changed.  I can tell this because I have seen tire sizes specified where the maximum allowable weight of the vehicle was exactly the load carrying capacity of the tire - AND - I have seen tire sizes specified that had in the range of 40% extra capacity - AND - I have seen vehicle models where the tire size specified was larger than the earlier model.

But because this is such as complicated thing, the US government requires that every vehicle have a sticker on the vehicle - commonly called the placard - that states what the original tire size was and what pressure the vehicle manufacturer is specifying for that size.  Match the load carrying capacity of that tire / pressure combination and you will be in safe territory and you don't need to know how the vehicle engineer got there.