John,

I would agree with Jorell. I swapped a 3.31 into my son's 83 244Ti w/ AW71 in an attempt to get better MPG and make the engine less busy at highway speed.
It calmed it down on the highway but I've seen only about 1-2 MPG increase with the 3.3 rear end. Off the line performance really has suffered. I just pulled the 3.3 and installed it in my 86 244DL w/M46. The 86 original 3.31 rear end was damaged and this is a direct replacement for the old diff. I'll going to try a 3.73 as a compromise between the original 3.9 and the now removed 3.3 on the 244Ti. I think the 3.73 is about the best choice behind the AW, at least for N/A 240s. A good tune up, clean air filter, properly inflated tires, real synthetic oil(engine & rear end)and ATF will probably get you more MPG and certainly demand a lot less effort than a rear end swap.