Almost certainly different differential ratios. You'll probably be happier with the diff from the euro car as well. Swap in the speedo from the euro (which is geared to match the diff) and you're golden.

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC