I've hit 34.5 at 70 with ac on very little stop and go.. it's possible. the car has to be perfect tire pressure, no leaks and rpms. the sticker says 26 city.. I get about 24 regularly around town with over a half a million miles

yeah I'm not touching detergents. if you change oil often with good oil and you shouldn't have goo in your oil.

__________________
Only diesels in this driveway.
2005 E320 CDI 243k Black/Black
2008 Chevy 3500HD Duramax 340k
2004 Chevy 2500HD Duramax 220k

In my experience, this is the wrong way to drive a diesel. This driving style (in my opinion) will lead to carbon deposits in the pre-chambers (and more carbon in the oil = more engine wear). As the carbon builds up, combustion efficiency goes down, fuel consumption gets worse, more carbon builds up because the combustion process is less efficient, and the viscous cycle drags the car down.

If you gave the car an Italian tune-up right now, would it belch smoke as the carbon burns out? If yes, then I am sure that this is part of the problem.

I would say stop the babying and mash that accelerator down whenever possible. Keeping the RPMs so low is probably not helping at all. Diesels are VERY different from gassers, engine efficiency has more to do with load and less to do with throttle position. Not familiar with the electronic transmission, if she can select "3" instead of "D" and force the trans to stay in only 1st, 2nd, and 3rd gear (while in city traffic) then she may see an improvement in mileage. This is counter-intuitive, but worth trying.

How often does the car get an Italian tune-up? I try to get at least one or two every day (once the engine oil is at full operating temperature).

I get 24-25 mpg in city driving, but not all heavy stop and go like what you describe, I've got a couple highway jaunts in my commute with speeds up to 70-75 mph, with opportunities to burn out the carbon. This mileage is in my 124.193 (OM603) that has an injection pump from a 3.5 liter engine, and I haven't yet turned down the fuel so the governor is over fueling my car. In other words, I could probably get 26 or 27 mpg if I tweaked the pump governor a little bit.

Last thought: Does the engine get up to full temperature quickly? If the thermostat is bad and prevents full operating temperature, that can also kill your mileage. I just did this on my 124.193 a couple months ago, and gained about 2 mpg as the carbon has been burning out. Old t-stat was keeping the temperature right at 80 deg C in the cooler weather, new allows temp to stay around 90 deg C, and now my mileage is better.

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC

Very short trips on steep hills with many intersections along the way over the span of several weeks.

__________________
CENSORED due to not family friendly words