Well, finally after changing my rims back to the OEM and accurately measuring the circumference of both tires at 36 psi and OFF the car I am surprised by my findings. OEM 18" rims with 335-30-18, the circumference is exactly 82.5". Then measuring the 19" rims with 335-25-19, the circumference is exactly 82.5". Therefore the aftermarket seller of the rims CORRECTLY UPSIZED the tire/rim combination. I hope this helps someone. Since the circumference is egual to pie times the diameter, the diameters have to be the same.