I have both in 123 chassis (3.07 in the 82, 2.88 in the 85).

as others have said, 2.88 cruises at about 65 mph @ 2800 rpm, 3.07 is about 3200 for 65 mph.

its a difference mostly in noise. maybe 2-3 mpg difference, but its barely noticeable.

as having both, i personally wouldn't seek to swap to a 2.88 from a 3.07 unless there was something wrong with the diff and it had to be swapped anyways.

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car