Took a 5mpg hit the last two tanks, car runs normal, girlfriends work car.
Started about time temps really got winter like here (oklahoma), premium non-ethanol always same station. Her commute is kinda short and unfortunately she has to run two kids in two separate trips each morning a quarter mile to the bus stop and back, then leave for work an hour later. Sort of a worst case scenario but such is life. Should I be concerned or just the nature of the season? My first winter with it, gone from 22avg to 17.

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One more Radar Lover gone...
1982 VW Caddy diesel 406K 1.9L AAZ
1994 E320 195K

Depending on the weather, the fuel blend, the oil you run, and your parking conditions, 5mpg can be a normal MPG penalty.

For example, if it's below freezing in the morning, the car's been parked outside, and you're running 10W-40 oil, you can expect the car to run rich until it reaches operating temperature and have additional losses due to the viscosity of the oil. Before tearing into anything, wait for better weather and see if your economy returns to "normal".

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Current stable:
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1983 500SL 120K (SLoL)

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Roger that, kinda what I was thinking as it feels as sprightly as ever when warm and it spends an unfortunate amount of time in cold enrichment mode I'm sure.

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One more Radar Lover gone...
1982 VW Caddy diesel 406K 1.9L AAZ
1994 E320 195K

Don’t neglect the thermostat. Old ones tend to leak and prevent the car from reaching its full 87C op temp in the winter. This can also lead to condensation in the oil.

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Prost!

I have a 78c thermo,fuel blend is **** in winter anyway.M104 prone to head gasket failure is why I run it cool

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1999 w140, quit voting to old, and to old to fight, a god damned veteran,you are a citizen of the world, all peoples are
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New OE Mercedes one installed last spring when I replaced the radiator, seems to warm up quickly and reach good temps per the gauge.

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One more Radar Lover gone...
1982 VW Caddy diesel 406K 1.9L AAZ
1994 E320 195K

Does the driver start the car and let warm up for more than a minute? Warming up a car long term = 0 MPG

I recent got a non MB with an average MPG readout. It had last been driven round October 2017 ( warmish weather ) . MPG reading was 23.8 , I started driving it early Jan and MPG started to drop instantly. After a few weeks of 0 to 7 * F mornings and 12 * days. MPG settled out at 18.5. Now that it is getting warmer, MPG has risen to 18.9.

A very cold engine takes longer to warm up so it spends more time in fuel enrichment. Oils are thicker causing more drag, especially rear end oil. Driving through snow creates a resistance too.

Running the rear defroster? Heater blower on high? These take more energy as well.

I'm sure she does, and like I said the poor car ferries two kids, in two separate trips, about a block down the road and back every weekday morning. So there is cold enriched operation a plenty. I don't think I've ever seen a car take such a hit in MPG over winter is why I asked, but I've never used one such as this one is either, so.

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One more Radar Lover gone...
1982 VW Caddy diesel 406K 1.9L AAZ
1994 E320 195K

My MPG rose to 19.2 at the end of 20 miles of mixed driving, AM temp was 46. Pm temp was 56 but return trip only netted 19.4 probably due to 2 stops along the way.

On your end, hopefully the work drive is long enough to boil off moisture and charge the battery from the 1 block drive.

Yeah she has about a 13 miles each way that's partly in town and partly on the highway to work and back every day. I still stick to 3k intervals but it gets plenty warm. In fact it's impressive how quickly that car warms up, she's commented on it many times. Very nice in the winter. Takes my huge under-stressed low rpm v8 Lincoln forever to heat up it's various juices even with a new thermostat.

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One more Radar Lover gone...
1982 VW Caddy diesel 406K 1.9L AAZ
1994 E320 195K

The 17mpg average sounds reasonable given your description of how the car is used. Winter blend fuel has slightly lower energy content per gallon, but most of the impact is because the engine spends so much more time running rich before it warms up, and folks spend more time getting 0 mpg letting the engine warm. The 124 petrol cars were never spectacularly efficient either, just ok wrt fuel economy.

Thanks all, sounds like I can safely ignore till warmer weather comes back.

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One more Radar Lover gone...
1982 VW Caddy diesel 406K 1.9L AAZ
1994 E320 195K

Though the coolant warms up quickly, people that have probed the oil temperature find it takes around 9 miles for the oil to fully get up to temperature.

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95 E320 Cabriolet, 159K

Always takes oil awhile, I have an oil temp gauge in one of my other cars and use it to gauge how ready for "normal" driving it is. Can take a long time with a lot of oil, 10 minutes depending on the use isn't all that long really.

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One more Radar Lover gone...
1982 VW Caddy diesel 406K 1.9L AAZ
1994 E320 195K

With continuing 45 ish weather at work MPG was 19.5 , at home MPG 19.8