Bad O2 sensor?
It sounds as if you have a rich running condition or transmission slip, dragging brake, etc. Could be a number of things, but I'd check the tailpipe first -- if it's black and sooty, your running rich and need to check the fuel system.
Rich running could be lots of things -- leaking cold start valve, leaking injectors, bad O2 sensor, bad ignition wires or plugs, etc.
Does it still run and idle properly, or is it rougher/slower than normal?
Just changing to Sport mode and back shouldn't increase the fuel consumption THAT much!
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!