I don't believe there were any W123 cars that used 2.47 differentials. I also do not think you can use the W126 differential in a W123.

How much of an increase in your mileage are you expecting to see? If you think you are going to see consistent 30's then save your pennies.

Please reference my mileage chart at www.pokrefke.com/benz/benzmileage.htm (Note: the diff was changed from a 3.07 to 2.47 in February 2006)

Using the stock 3.07:1 differential I drove 12,209 miles and used 468.7 gallons of fuel for an average fuel economy of 26.04865 mpg.

Using the 2.47:1 differential I drove 11,791 miles and used 426.9 gallons of fuel for an average fuel economy of 27.62005 mpg.

The economy increased 5.69% or 1.571406 mpg using the 2.47 differential.

Although the engine does turn slower and makes for a much more quiet car, you are just not going to see a huge gain in economy.