Get up in the morning, when the temperature and tires are both cool. Check the tire pressure. Read the side walls on the tires, and fill/ deflate them to what the tire says. Make sure you do it when it is COOL. When the tires are hot, the pressure rises, and gives you a false reading. And remember it is always better to be a couple psi over, than 1psi short (unless your dragging/ racing or something).
edit: if they still wrong, you could have bad tires/ sidewalls etc. get them checked
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