Need advice on differential ratio for a w123

[ivanlee190e]

Hi all, I have searched before and I found a wealth of information that is too much for me to absorb. This is why I am starting a new thread on this. I have a w123 1983 300D Turbo Diesel and I am interested in lowering the RPM by changing the differential ratio. Currently I have the stock differential and I have realized that some other models like the 1980 280 can provide some relief. Are there other years and/or models that have a differential that can provide me the necessary ratios to lower the RPM say at 75mph, so I can get better mileage. I have tweaked the turbo and opened the ALDA so that I am now getting way more horsepower and can pull those gears. "I NEED HELP PLEASE."