I wondering what my options are... currently I have a 2.47 diff installed in my 300CD (turbo). I'm not sure where it came from (W126 or R107) but it’s an open diff with a 210mm ring gear. We had to swap the diff cover from the original diff for it to bolt into place.

Right now I feel like the ratio is a little too low. While on the highway the revs are low... the motor seems to be lugging a little. In 1st, 2nd and 3rd gear the car moves decently, in 4th it really struggles to gain speed till you get to about 3k RPM. Also my speedo is pretty off now.

I'd like to keep this larger diff as I have a 560SL diff (2.47 LSD) that I'd like to use. However what are my options in terms of diff ratios that I could use in this diff?