i have 13 inch alloy but not common alloy .. if you check new alloy rims these days in shop you will find them so heavy .. i bought used one but they are a lot lighter than new chinese alloy rim and i am using tyres 165 / 65 / 13
I think for stock rim tyres should be 155 / 70 / 13 which means tyre will have more profile and your car will slightly get more height and may be it will improve smooth drive because some times i feel stiffness with 165 / 65 / 13 tyres in my car..
in the end you should have better milage with stock rims