Hard if not impossible to top the Mutt.
BTW around 25/mpg on my 23 mile county road commute 85/ 617.

25.9 mpg over a 5,000 mile trip..........
 

.............from San Francisco to Maine via Grand Tetons, Yellowstone, "Top of the World", Cody WY, Little Big Horn, UP Michigan, Cleveland etc. etc. etc. Best road trip so far on every kind of paved road you can imagine.

1984 300TD with 250k plus miles, added less than one quart of oil to keep it halfway between the add and full notches. I used additives by Archoil to both the oil and fuel. A good buddy who works for a recent Tonight Show host told me about this stuff.

She was fully loaded too by the way, had to make room for the bride in Traverse City:D

I get 28-30 in my 4 speed 83 240d swapped with an 85 om617 and the 2.88 diff.
I get 24-26 in my California spec 85 300TD.

All my mileage is mostly highway.

23.5 mpg around town in my daughters 1981 240D with a manual. This car is in ridiculously stock condition.

I took the the old sled on a short road trip once, 28.5 mpg at 63 mph.

82 240D 4 spd. 28-32
82 300TD 4 spd. 26-29.

I get between 27 and 29 in my 1982 300CD.

I don't know if you guys like Bill Burr, but here's his take on MPG.
https://www.youtube.com/watch?v=CX0MnVQTGPw

I can hear many opinions on big oil and auto company conspiracy theories without this guy who feels that the only way to make his point is to swear constantly, I'll bet he uses the F-word alone over 100 times in this constantly rambling rant.

Thumbs down.

Sure, it is believable that automakers don't want to invest in better mileage if it costs them money without return on investment, and that oil companies want to sell more oil, ... duh. But the point that this guy makes on cars getting the same mileage as the 1970s, ... hardly. A full-size Cutlass Supreme (for example) or Monte Carlo would return high teens on the highway, we used to run synthetic oil, Michelin's best radials, premium fuel, and 55MPH to get any more than that in a car with less than 300hp. Today we're driving 75+mph in 300hp V-6s and still getting low 30s (my recent round-trip to Cleveland for example in a rented Challenger returned 31.9mpg on regular, including a week of in Cleveland driving). I'm willing to bet that driving that same car/trip at 60mph would be 36mpg. I usually rent a V-8 4WD Chevy Tahoe, and average 21.6mpg at 74mph, that same vehicle in 1977 would have gotten around 10mpg (I had a much smaller/lighter 302 automatic Bronco, 12mpg was my average).

So maybe they could do better if the ROI was there, but it isn't, and same mileage as '70s is pure ignorance. Further, if you required the emissions levels mandated today, the 1970s cars couldn't even come close.

If fuel gets back to where it was 10years ago then people will start buying on fuel mileage again, today it's all SUVs and large pickups.

And not really on topic, but 2015 ML bluetec 4matic mixed city/highway since new average (13,000miles): 32.6mpg (calculated from fuel logs and GPS verified).

1984 Auto 300D NA with aircon and SLS suspension. European model.

Measured the following:

1) 29mpg in the city.
2) 42 mpg on the UK motorway at 60mph or less.

US gallons?

No imperial gallons. US numbers are 25/38.

I would guess that the ultimate 123 mileage champ would be a 240d with a 5-speed manual and a turbo. I think there were English market 240d turboes... Maybe with the axle ratio from an 85 300d Otto-matic. And 175 or 185 tires. But it wouldn't be a fun car to drive, or even get very good mileage. My wife's 2003 VW Jetta tdi wagon really never gets less than 35 all around, 45 mpg on the highway.