does anyone know if a 56 plate 150 bhp vectra c 1.9 cdti six speed gear box is the same as a 54 plate 120 bhp 1.9 cdti vetrac design six speed. as the 120 does 71 mpg at 60 mph and the 150 does 55 mpg at 60mph so if i get a 120 gear box and put it in a 150 car i should get realy good mpg ?