W123 change trans = improve mpg?

[joeywarren]

Hi all - I have a UK W123 230E auto 1983. I'd like to improve the mpg. Other than going *everywhere* at 55mph ;-) is replacing the rear axle/diff with one from another merc a good way to do this?

I have heard that the auto 4 speed suffers by 15-20% mpg as it has such a high final drive ratio!

What if I replaced from a manual 5 speeder? Or from a different W-series altogether? Am I mad?

What is involved in changing the ratios, does the trans need to be removed, or can parts be sourced? Novice!!

cheers