wondering if smaller rim size (15) on my 2002 impreza obs will screw it up in any way. The mechanic doesn't think so as with the tire size will make only .18 inch difference.

As long as the tires are the same all the way around, and the 15's clear your brakes, you will be fine. The only difference is your speedo will be a little off, but they are usually off stock anyways, so it really won't matter too much. The only thing you have to worry about is if you use different size tires for some reason.

-Wes

thanks. New to owning cars but eventually looking to work on it. Starting to learn but better to learn by advice than trial and error.

Just make sure when you downgrade, you increase the profile and decrease the width of your tire. There is a formula out there, and as long as the final circumference is very close, you won't notice the difference in speedo reading (not saying it's accurate, but it's close enough)