Tire pressure with plus one sizing

[C M Smith <cmsmith@domain.elided>]

In the hot air department, I can be counted on to chime in.

I notice that the specified tire pressures for my recently acquired SAAB 
9000 Aero with 205/55x16 tires are about 5 psi higher than the spec for the 
195/65x15 tire specified for the same car in other versions, no weight 
differences, and when fitting 195/65x15 winter tires to the Aero, the 
smaller tires are also spec'd with the lower pressure. I infer that the 
lower profile tire benefits from higher pressure, when used on the same 
vehicle. I suspect that one could easily prove this geometrically, if one 
remembered how to do high school geometry, perhaps calculus would make it 
easier.

If the new plus one or plus two tire/wheel combo has the same rolling 
diameter (as it should if done correctly) then the wider tire with the 
shorter sidewall should have a larger internal volume. As the car is held 
up by the deformation of the tire at the ground, and this deformation is 
proportional to the initial tire pressure, then the change in volume of the 
larger tire will represent a smaller proportion of the total tire volume. A 
higher initial pressure will be required to ensure that the proper tire 
profile is maintained at the contact patch when the tire deforms by the 
correct amount to support the car. The actual area of the contact patch 
should be approximately the same for each tire choice, the shape will be 
different, the lower profile tire having a shorter, wider patch.

How much extra pressure would depend on the tire construction and the 
precise difference in volume between the stock tire and the plus one or two 
tire, the manufacturer of the tire will have the best information and this 
will probably relate to the weight and distribution thereof rather than the 
model of car.


Michael Smith
Calgary, Alberta
Canada
91 Alfa 164L, White, original owner

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