Any Indie shop should have an analyzer and should be able to dial you in for much less than the dealer. Will probably take about 1/2 hour to dial it in, it needs to be checked at both high idle (2500 RPM) and low idle (600-800 RPM).
The 12 MPG, is that city, highway or combined? On the 560SEL I get a shade over 13MPG city and 19+MPG highway (extended trip).
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.