in my opinion based on wat u say ....4th gear 80 kph i tnk its normal..sagad mo 3rd gear mo hngan 100 up..saka ka mag 4th gear paps my ibubuga pa 3rd gear mo eh gagapang yan 4th gear mo kaya my parng hangin n sound knina try ko use lahat gears with low speed like 70 80kph un nga mahina hatak i tnk un ung nrrnig mo na hangin hirap lng engine nun kc naka 4th gear ka agad try using 3rd o 4th gear sa simula plang as in full stop kung tama ako eto prn mrrnig mo..come to tink of it 120kph mc mo i tnk every thng is ok naman paps