I just don't think you can reach that kind of mileage with a heavy brick driven by a Hitler era diesel.

You would have to reduce the weight considerably by cutting the car in half so there is no back seat area, chop the top for wind resistance. Section the car into a wedge. Replace the bonnet, wings & boot lid with light fabricated fiberglass. Get rid of the bumpers. Forget the amg crap; make your own street scraping air dam.
High gear ratios with super slick lube and then use hyper-mileing driving techniques.
Just get a vw jetta diesel or a Honda crx or hatch back Honda vx and enjoy life.:D

i think youre giving up too soon. i think with driving technique and a few changes you could get in the 33-35 MPG range and still have something safer than an econobox will offer. If you want to spend 10-15k for a VW TDI then payback will take a pretty long time.

IMO, the most practical change would be to swap the differential. I'm still thinking about putting a 2.88 in mine; partially for mileage, but mostly to reduce the rpm at highway speeds. These cars were really geared for the 55 mph speed limit and they are just a little too buzzy at 75 mph.

slowing down is the biggest improvement, you will see immediate improvement dropping from 75 to 60.

next is switch to a manual trans, without a lockup converter there is always some loss of efficiency, and the 4 speed is significantly lighter

i am working on two options for gearing. the first is a 5 speed as this gives overdrive and a good first gear. the other is to drop the rear gear ration to get the final drive to be the same as an overdrive when in 4th gear. drawback to this is first gear becomes rather tall and hard to take off. for those in other regions of the country there would be a lot of shifting even on the highway. Since I am in florida there are no hills to speak of so this is not a big issue for me. I have a 2.47 I pulled this weekend, but I have a 5speed option in the works right now so I am goig to wait and see what developes

ok, I am working on building a wind tunnel... really... a VERY small one, JUST big enough for the doors and roofline of the car to fit in and not affect the particulate of the vehicle. I am building in in a venturi shape so air velocity is not required by the fans. two shop fans mounted in a rear opening with a venturi shape where the car sits, will make the air flow fast enough around the car to create turbulence. it's quite a ways down the road right now though...
the 123 is quite a brick, but it does have certain elements that can show improvement with minor adjustments.
first look at the bumper, and lose it.
next put in a nice roll pan up there to blend in with the grille. incorporate a low air dam to it to push as much air out from the front to the sides as possible. this will also get you added downforce if you plan on breaking 100 mph sometime...
ok, next focus on surface airflow. you will need to remove ALL windshield trim and get the surface of the car as smooth as possible, take out the windshield trim and gasket and set the window as flush as possible with the cabin. smooth it with urethane. get rid of the wipers, mirrors, side molding.
the back need a bubble where the trunk meets the rear window. think of a bubble that carries the roofline in a smooth arc all the way around the back to the tires in a boat shape...
report back when this is done...

I agree, but neither of those is going to happen. I don't usually have time to drive at 60, and I'm not interested in replacing my transmission (or driving a W123 with a manual).

I used to say the same thing. as it turns out I am much happier now. I used to have moderate road rage becuase I just could not understand why people wont move out of the way in the fast lane. i would swerve around them an d go just as fat as traffic would allow, sometimes faster. now I drive 60, let the type A's pass me and in my 32 mile each way commute I notice absolutely no difference in time. my life is much better and calmer than ever before.

I drive some pretty long distances, In the last two months I drove from CO to GA to NC to CO and made two additional round trips from CO to NC. That's about 9-10K miles, so that speed difference would be about 30 hours out of my life.

i drive from Tampa to Cincinnati 4-6 times a year. going fast as I could, blowing through traffic changing lanes my best time was 12-3/4 hours. driving the speed limit all the way i make it in 13-1/2. trust me it was a much more pleasant trip.

Most of the places I drive have a 70-75 mph speed limit, and much of the traffic travels at 80+ mph. I normally drive within 5 mph of the speed limit. Diving significantly slower than everyone else on the road is also stressful (and dangerous).

Hmm, assuming you travel 80MPH and figure the 10K miles.... that's 41.666666666666666666666666666667 hours of your life you would be in the car on the commute actually... drive time that is...

10000 (miles) / 80 (miles/hour) = 125 (hours)

ok, thats a different story. for me it was just go as fast as I could. my dialy is in town so a mixture of 55-65 limits. when I go to Cincy, i usually run 70 for the same reason you stated above, but I used to do 95 as much as i could and it did not make a bit of difference in time of travel, but my state of mind was so much better.

I agree, the easiest driving is when you are traveling about the same speed as everyone else on the road. Driving very fast is more trouble/risk/expense than it's worth.

yup, and the same 10K at 60 mph is 166.66666666666666666666666666667 hours... or 41.666666666666666666666666666667 more time...