And how long does our guy let the car warm up? Idle = 0 MPG

I have a Chrysler product with an average MPG display, in winter the MPG drops 3 MPG running the same route vs middle of summer. I don't let the car warm up more than 30 seconds or so in the winter.

Snow increases tire to road drag, tire pressure drops causing more tire drag, oils are thicker causing more drag , engine takes longer to warm up , rear defroster takes electricity ( though this is somewhat offset by the rad fan running less ) , heater blower might be running longer / higher speed taking more electricity, maybe more time sitting in traffic due to road conditions.

Same thing happens with my 606 at 91.000 miles. Summer or winter - it makes no difference, I get 8,5L/100km at 70mph on tempomat while other owners report 6,5.
Everything seem to work fine on the car.

Look under intake manifold and see if there are any leaks.

I vote winter blend fuel 1st.... then see if any leaks

99 e300td- gets around 30 mpg... Only driven in summers -92k miles

my 05 cdi-- 31 mpg winter... 34 mpg summer-- 39 mpg all hgwy

87 sdl-- 29 mpg Only summer...

No leaks, when I put new orings I also replaced the rubber fuel lines. Everything is dry.
I drove it across Europe and fueled in many places. Last summer it wasn’t doing any better. I bought it with 82k on the clock

I might pull the injectors and have them checked. Standard Mercedes maintenance at 100 thousand miles. Cat deterioration as mentioned I think was a good thought. Anyway to check them other than as mentioned?

Air filter, alignment, tire pressure, outside temperatures, speed will all make a difference. For winter a big factor on the highway is that the air is more dense so it requires more energy to move it. Do you give it the old Italian tune up and go full throttle to high rpm? That helps too.

Diesels can't run rich like a gasser can so if you are not being followed by a plume of smoke you are burning the fuel injected properly. Squirting injectors, dragging brakes, plugged air filter, etc. etc. can hurt mileage a bit but only on the margin. I typically get about 23 in normal driving and about 30 on a trip in my non-turbo 606 which is geared down to make up for it's low power. Make sure you don't have a major fuel leak that you are not noticing. I suspect your mpg calculation technique is your problem.

I would like to "check" the cat by removing it if it's going to make a difference. I don't have an injector pop tester and the original one is not cheap. I should send them to a workshop...

I fitted a new air filter just in case a while ago, nothing changed. I use 4 new Michelin Alpin 6. Sometimes I kick it a little but I don't think it was more than 100mph+

There is no smoke behind this car no matter how I drive it. It rolls fine if I put it in neutral before a red light and calipers are "cold" (I can touch them). Before I left Romania for UK car was lifted and I removed the shields in order to check its overall condition. Apart from the shaft drive bearing - the rubber is a bit tired all is nice and clean. I record the miles/liters when I fill the tank. In almost 10k I had time to check this several times.

News: I bought the parts and fitted the EGR defeat system. Car feels more responsive, I wonder how the mileage will be influenced. Still no smoke.