I have a faint recollection that Mercedes attempted to enhance the fuel economy in their M117-engined cars for the 1980 model year by detuning the engines slightly and making the final drive ratio a bit taller than preceding years. These changes did not enhance the performance of these vehicles. I would go for a 1979 model over 1980 for this reason, unless I had a good drivetrain from a 560SL laying around.