What year civic ex??? that matters a ton, 93-95 are the lightest and stock should run high 16's, probably 16.5 - 16.9, 96 or above stock probably low 17's to mid 17's, and theres no way with just and I/H/E you would run 15's. never seen it happen and i got to races.... but maybe you guys have a good good good good good driver, i mean better than professionals..but no way 15's...sorry but I/H/E won't give you that much faster.