effect of rearend gearing on ET's

With mph at 115 that should have you in the11,s.mine runs108 mph which i should be in low 12,s. See,in I ran 12.8 at 108 my mph shows low 12,s this is with some traction lose and a 4 speed.I,m upping my gear from 4.10 to 4.57 and hopefully gonna run high11,s with some tweaking and practise.Your destop e.t,s don,t sound right??115mph in the 13,s ??I,m confured.