Originally Posted by
SoreFeet
The problem is you can't see how the inside fibers are wearing, thus you have a time bomb. However if the bike is primarily used on flats and basic transportation without doing huge hilly descents I'd ride it till it blows. Just repair the inner casing with some shoe goo or a tire boot.
for me it depends on how much tread is left. with a gash like that i'd replace the tire if the tread is really squared off but if the tread is still a bit concave i'd repair and ride without hesitation -- especially so for a rear tire.