I believe it is Zebra string is made out of a fast flight type material (therefore will stretch slightly more but also more forgiving) and Tiger string made out of BCY 450 Plus in 1/2 size (thinner 450 Plus so there are more strands but of a "non-stretch" material therefore slightly slower and less forgiving). I would assume that the speed difference would be under 5 fps. The way it was explained to me about the speed difference is the fast flight types will stretch a little and lengthen you string (which ups the weight) enough to gain some of that speed plus they are slightly lighter.