15 is the wheel dia and 6 1/2J is the wheel rim width (both in inches), (the 'J' refers I think to the bead style that retains the tyre). As long as the wheel bolt PCD is the same as the original wheels (100mm x 4) the things that you will need to check are the wheel offset which means the distance from from the hub location face of the wheel to the inner rim face of the wheel should be the same or similar so that the inner face of the wheel/tyre does not foul on the cars suspension when fitted (this can be remedied though with hubcentric wheel spacers), but also what is important is that the spigot diameter (the hole in the centre of the wheel) is the same as the original wheels. Too small and the wheel will not seat down on the hub and too big and you lose the concentric location that keeps the wheel central on the hub, and NO the wheel bolts do not do this effectively! If the spigot diameter on the replacement wheels is too small a competent machine shop can machine them to the correct diameter, for a price.... If the spigot diameter of the replacement wheels is too big then plastic spigot rings can be used to get the correct fit.....