I don't have a 123 but my 87 w124 has never gotten less than 30mpg with a best of 34. The 30mpg was a tank full of 85 to 90mph drives though my 70mile one way commute. The 34 was doing 55 to 65mph depending on speed limit.

I recently started to record the mileage.

https://www.fuelly.com/driver/zorecati/300d?fu=5523902

One of these days I should really try driving that slowly for a whole tank, I've heard great things about it. Unfortunately 55-65 around here relegates you to the slow lane where the pavement is full of potholes from heavy truck traffic. Those same trucks will also be tailgating and passing you on the left.

+1 on Fuelly. Surprisingly few 300d drivers are using it. Its an excellent tool to track mileage, cost per mile and I also use it for oil change intervals.
Mercedes-Benz 300D MPG Reports | Fuelly

__________________
CENSORED due to not family friendly words

Here's my mileage on a recent coast-to-coast trip in October (San Diego to North Carolina) This is my 107 with a 617.952 engine, 722.3x transmission and 2.47 rear end. The 107 has about a 200# weight advantage, but with luggage and 2 passengers, some of that was likely countered.



Note that my odometer reads 10% high so the mileage numbers are raw while all the other numbers have the correction factor built in.

Wind, altitude, braking, speed all have a huge impact on mileage.
The 2.47 rear end probably helps a lot- 2600 turns at 70mph. My "normal" everyday around town is usually around 29.

__________________
Current Stable

• 380SL (diesel)
• Corvette C5
• Manx
• Baja Bug
• F350 Powerstroke
• Auburn Boattail Speedster replica

OK. So let me get this straight... you have a diesel SL? That's pretty sick. All that stuff about the gear ratio is over my head though. Did you use the original turbo?

Search mach4's username, he has a fabulous thread on the conversion of his SL. Worth the read

__________________
This post brought to you by Carl's Jr.

Hi Chris!

The car is originally a 1983 240D non-turbo with an auto trans. I changed the gearing to a 3.07 diff out of a 300 turbo, which dropped my cruise rpm about 600rpm IIRC, so its turning about 3200rpm at 75mph. I'll re do the math if you would like. I also swapped in the four speed manual trans. I manufacuture an adapter plate that allows puttin a 5 cyl turbo manifold off a 617 on to the 4 cyl 616 which makes the conversion as easy as bolting together lots of junk yard parts, so thats how the turbo got on there! I have pics in my old posts. Since then a different air filter set up is used and I've improved the oil separator. I will post some new pics this weekend after the whole Thanksgiving family drama is over!
Cheers
chris

I forgot to mention the pickup is quite good! IP is also um......not stock.

What did you do to it? 617.95 elements?

__________________
"Senior Luna, your sense of humor is still loco... but we love it, anyway." -rickymay ____ "Your sense of humor is still loco... " -MBeige ____ "Señor Luna, your sense of humor is quite järjetön" -Delibes

1982 300SD -- 211k, Texas car, tranny issues ____ 1979 240D 4-speed 234k -- turbo and tuned IP, third world taxi hot rod

2 Samuel 12:13: "David said to Nathan, “I have sinned against the Lord.” And Nathan said to David, “The Lord also has put away your sin; you shall not die."

That is pretty cool right there. I wonder why Mercedes never put a turbo on the 616.

__________________
Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 401,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 26,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.
99 Mazda Miata 183,xxx miles.