'79 300SD, great compression, valves adjusted regularly, timing chain within specs. Idle, rack dampner, linkage all correct. No fuel leaks anywhere. Clean air filter. Runs great. Starts easily. BUTTTT....in the last 12 months, the fuel mileage has gone from 25MPG down to 14.5 as of today. It has been a steady decline, 25, 22, 21, 19, 18, 14.5. The car goes really fast. I have used injector cleaner on a regular basis. No smoke, regular oil changes. I drive a combination of city, and highway, nothing different recently.

I suspect that the metering in the fuel pump has failed, or the nozzles of the injectors are worn.

Does anyone have any ideas about what would cause such a progressive loss of fuel economy?