Tires: Formula for Tire Pressure?, tire placard, inflation pressure

Question
Is there a formula for tire pressures (weight of vehicle, weight on axle, size of tire etc.) that auto manufacturers or tire makers use? I know that a larger tire requires less pressure for same load. I recently installed larger tires on my smart car but am curious about lowering the pressure correctly. (175/55-15's replaced with 195/55-15).  

Answer
Don,

There isn't a formula, but the technique involves the use of load vs inflation pressure tables published by the tire standardizing organizations.  The starting point of the calculation is the tire size and the inflation pressure listed on the vehicle tire placard.

On every vehicle sold in the US - and I understand this is a worldwide practice as well - there is a sticker – commonly called the vehicle tire placard - that lists the original tire size and the proper pressure for that size.  The placard is usually located on a doorpost or in the glove box – but sometimes it is located in the trunk or on the fuel filler door.

BTW, it doesn't matter who makes the tire or what pressure is listed on the tire's sidewall, if the tire size is the same as the placard, then the pressure listed on placard is also appropriate.

But there is another problem:  Smart cars had some problem during their development - instability during hard maneuvers.  As a result, the cars are specifically designed with different tire sizes (and different wheels) front to rear, to improve this instability.

The key points are:

1)  The front wheels are too narrow for tire sizes that fit on the front and the rear wheel are too wide for tire sizes that fit on the rear.  I hope you took rim widths into consideration when you made the change to the wider tire.

2) There is a large pressure differential between the front and rear tires.  So any change in inflation pressure ought to also keep the same difference in inflation pressure front to rear.

So I need to whole picture to answer your question:  That is, what does the vehicle tire placard say for tire size and inflation pressure - both front and rear.  I'll also need to know what you did relative to the rims, and what you've done about the front and rear tire size difference.