hi does anyone know if a vectra c 56 plate 1.9 cdti 150 bhp six speed design have the same gear box as a vectra c 54 plate 1.9 cdti 120 bhp speed design, the reason i ask is the 120 bhp at 2000 rpm at 60 mph is doing 71 mpg the 150 bhp at 2000 rpm is doing about 57 mph and is doing 54 mpg acording to the trip computer. any one explain please......