Tires: Tire PSI, tire placard, copywrite laws

Question
I recently had 2 tires replaced on a 2005 Mazda 3 hatchback.  I am getting ready for a road trip and checked the air pressure.. The two new tires say max PSI of 50, the two old tires say 45psi..  I filled two new ones to 50, and the old ones to 45, and now the check tire pressure light is on..  (when I first checked the tires they were about 35 PSI).. so my question is, is it ok to have 2 different PSI, and do I go with the max PSI listed on the tires even though the idiot light came on?

Thanks!

Answer
Kara,

On every vehicle sold in the US, there is a sticker – commonly called the 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.

If you are using a tire size that is different than what is listed on the placard, then the pressure has to be recalculated.  The calculation is not difficult, but it requires tire load tables which are not allowed to be published in the Internet because of copywrite laws.  However, I have a copy and would be glad to do the calculation, but the starting point is the vehicle placard: Tire size and inflation pressure.  I will also need to know the new tire size.