On a mile trip if I warmed the engine up for a minute at the end of a mile my engine would not have made normal operating them yet. If the Engine is not up to temp part the energy from combustion is going to warm up the coolant and not burning as well and that energy is not going to the Wheels.

Then there is Winter Fuel which gets poorer mileage.

I would also be suspicious of 221k original injectors. I would gestimate that at 100-125k the Injector Nozzles would be at the end of their life and that the Pop/Opening Pressures would be off.

Once in awhile you need to take the Car for a hour run on the freeway (when it is open to traffic). Short short trips you spoke of can cause sticky piston rings.

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84 300D, 82 Volvo 244Gl Diesel