FWIW, I just finished a 200-mile trip to Virginia Beach, in my '84 5-speed NA Euro 300TD. Well loaded, including a dishwasher. Average speed around 70 with the AC going. Filled up right when I left and again immediately upon arrival. At 6.6 gallons in 200 miles - I come up with 30.3 mpg.

Happy Motoring, Mark

I'm pretty consistent at 32-34 mpg with the 3.07:1 axle, but I have to keep my rpm's down for the best mileage.

This is why I chose the 2.47:1 axle ratio;

Currently, my best mileage comes in at 2450 rpm - 60 mph, but you CANNOT drive 60 mph on modern freeways, so I devised a plan to use a 2.47:1 axle based on the premise that it will allow me to cruise at 75 mph at 2460 rpm, which makes for more comfortable driving (in keeping with traffic flow) and keeps the engine where it wants to be.

The only downside I am sure will be the big hills, but that's to be expected in a big car with a 180 CID motor....

When i was still checking i was averaging 24/26 in average mixed driving in a 84 300sd. I have since changed jobs and have to burn through Detroit which seems to have caused the odometer gear to start slipping. So i quit checking. No matter how conservative i drove i never got better than 29mpg once.

My commute consisted of 220 miles a day round trip, all of it freeway, and I would burn on average 15 gallons to go 475 miles.

If I kept the speeds down to 60, I could pull 34 mpg out of the car like clockwork, but its not easy to run 60 on modern freeways...

I have tracked my MPG for the past year in my 240D and I think its fairly accurate. Leisurely driving most of the time on interstates and back roads with a few work commutes thrown in for good measure. Not sure what I'm doing wrong since all the cars for sale on ebay and craigslist say I should be getting 50 MPG ;)

25.5 avg MPG
lowest tank
23.3 MPG
best
27.4 MPG

4212 miles driven

My first tank following the 2.47:1 swap exceeded my previous best of 34 mpg with 37 mpg.

However, this reflected a lot of in-town driving, some freeway and running the A/C full time, so I will reports after 5 tanks of all freeway driving.

very pleased with the swap!

'83 300CD, 94 miles/days commute, 303,600 miles on the clock, normally run 65-68 mph at 7-8psi, 12-13 psi on hills, 195/75/14 Unicure A/S retreads on all 4 corners at 35psi, no a/c, windows down, no cruise control, 90-110 degrees outside, avg 23-24 mpg consistently. For the last month and a half, I've been siphoning fuel from my '87 KW cabover and '85 Cummins F350, but I'll fuel once in awhile at Quiktrip to check my mileage.

Our million km w116 300SD would always turn in 35mpg at 50mph on rural roads (speed limit verified with radar posts-the speedo shows 55mph). In daily highway commuting with the rat race 30 mpg +/-1mpg, a little less in the winter. It was really happy being retired and just slow cruising on back roads! We have owned since 1986, LOTS of fuel bills! I am just putting a 240D automatic on the road this summer, will see how it compares. Had a few w126 300SD that seemed good also, but usually the odometer was stuck on them so hard results were tough. All highway driving though, even the commutes were 95% highway with stop and go on the highway, very few traffic lights.
You can get good fuel economy way above the "norm", but not at the flow of traffic!

Used to own a very tired '82 automatic 240D that had been well thrashed by the PO's teenagers. Mpg range was 18 to 24, with around 21 typical in mostly local driving. Hope yours is in better shape.

Happy Motoring, Mark

The final results of the 2.47:1 swap is that my consistent fuel economy is 25-28 at 75-85 mph, where I got 25-28 with the 3.07's only at 55-60 mph.

My first tank did not yield 37 mpg, as previously reported, due to my own mistake in calculating my speedo error.

Not too bad being able to set the cruise on 85 with the A/C on and knock down 28 mpg...considering I am about as aerodynamic as a garage door....

Automatic transmission covering 7904 miles.
http://badges.fuelly.com/images/smallsig-us/208163.png

After 5 speed swap covering 2862 miles so far.
http://badges.fuelly.com/images/smallsig-us/208104.png

Every fill up was carefully documented at the pump. I carry two 20L jerry cans in the trunk which I fill up to the brim at the same time I fill the tank. This cuts down on trips to the pump and allows me to wait until I find myself near a station with the best price. Diesel prices can vary widely from station to station so I use http://gasbuddy.com/ to find the best price where ever I am.
You can see the raw data for each fill up if you scroll to the bottom of the links. The odometer and speedo have been checked vs 2 GPS devices and many mile markers. The vast majority of my driving is highway but I try to keep it under 75mph. I drive with traffic at all times. AC usage is about 50/50 depending on the weather.

If you don't already use fuelly, I suggest you do. Surprisingly few 300Ds.
http://www.fuelly.com/car/mercedes-benz/300d

In what chassis???? A 123????

I would believe that, but not in a W116...