15's on the Sonic, same size winter tires.
On my truck, I have LT tires, usually run pretty good numbers for pulling the trailer...like 60-80 psi (high enough anyway) - get it home after an oil change....32 psi. I don't think they can read, let alone believe they know what anything is based on. Throw a wrench at it and call it fixed. Charge you for losing their wrench.