Rule-of-Thumb for rim sizes: the rim should be very close to the same width as the tread area of the desired tire for best results. Narrower rims tend to pinch the tire in, thus not supporting the tread and giving an unstable feeling to the car. It also distorts the tread causing excess wear in the middle rather than evenly across the whole surface. Even wider rims are more prone to curb damage since the tire sidewall doesn't protect it from mishaps. More important than ultimate- width tires is trying to find a tire that is close to the original 28" diameter, so the speedo reads correctly and the engine is not over-revved at cruise speeds. Gas mileage improves over small dia tires, too.
