1989 should have an electronic speedo. I do not know if the odometer gearing is similar to the mechanical ones, but it definitely sounds like the gears are slipping. Have you timed a mile at 60MPH to verify the speedo is correct?
I was going to open by saying 40 miles to the tank is about right for a 560
, but decided that wasn't a nice way to welcome you. Bienvenidos.
'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.