You've eliminated a crucial part of the tire size.  Metric tires are sized
[P or LT]xxx/yyRzz where xxx = the width of the tire in milimeters at the
widest part, yy = ratio of the width to the sidewall height, R = the speed
rating (R is the most common for passenger cars good for sustained speeds
upto 85 mph) and zz = the rim diameter they fit on.  Assuming you're going
to replace P205/75R15 tires that will have an approximate overall diameter
or 27" with P235/75R15 with an approximate diameter of 29" then they
probably won't fit.  However P235/70R15 tires are only about 1" larger in
diameter and might fit.  A good tire dealer (not Sears or WalMart) should be
able to tell you.  They can also tell you if your rims are wide enough to
fit the larger tire.