300sdl the mpg website claims 18 n 22 i got 28.somethin from pa to vermont rarely goin over 65

if i take the motor(came outta 300d) out of my w126 300sdl n put it with the slighty different tranny(different gearing n speedo cable or something) of a 300d im sure ill get better mpg
or if it would work in an even smaller 190 id get evennn better mpg

thanks 4 ur input tho