As long as you can stay close in tire size, it doesn't affect your speedo and performance all that much.
I went from 225/70/15 stock tires to 235/75/15 on my Ranger (hint, there's a calculator for figuring out tire size, conversion from metric to standard and vice-versa in the tech pages on here), I think I lost a lil power but nothin to cry about, and my speedo seems to be reading correctly according to every police radar box thing I've gone past on the side of the road.
I'd just use the calculator to figure out what the overall tire size that you're running now is in inches and then try to find a similar size for a 15" rim. Or post up your stock tire size and someone here will figure it all out for ya.
