48MPG in an 87 300d - and no... not the cars for sale on Craigslist!
 

I know I'm going to get some 'Sure's on this thread, and no this is not another car advertised on craigslist stating 'twice the MPG of a normal MB' for the sake of a sale to an un-educated buyer. This is my car!!! It's a normal 87 300d. I did advance the timing before the experiment. I'm not sure how this affected the experiment. I'll have to do a few highway drives at 70 or 75 to see what my old baseline (28-30MPG) is with the new timing. Maybe that will increase too.

Anyways, I did an experiment after reading someone did this in a w123 and posted a thread titled "slow is the way to go". This wasn't a fun experiment, more painful than fun for me but I tried it and the pay off was very rewarding!;) Normally I get around 30MPG in the summer driving my w124 '87 300d with a normal stock 603 6 cylinder diesel with a normal load (me, my wife, and 3 little kids plus minor trunk cargo-Probably around 400-450lbs total). I've read this is pretty normal. But I also normally drive moderate to fast off the line and always go 75 or 77 on the Interstates here in Missouri. So... after reading that 62 MPH and under is much better on fuel since the cars were made to go 55MPH, I tried it. I made a drive 73 miles north (all highway) and kept my speed no faster than 62 MPH the entire way. At times hills would dip my speed to the high 50's and I didn't really give it much more right foot to compensate and just dealt with the slower speed. Most of the way my RPMs were around 2200. I tried to keep it at that or below to stay away from high turbo and IP boost (fuel consumption). Anyways, I drove like this for most of the 73 miles. I didn't accelerate hard from the one or two take offs on the trip. When I filled up at my destination I used 1.53 Gallons of fuel. Now...

73Miles/1.53Gallons = 47.7MPG

I couldn't believe it. The pump actually shut off before it hit 1.53 Gallons so I moved the car to another pump so the tank would be at an angle making the filler neck the highest point and pumped again. I filled it as full as I possibly could added the two receipt's gallon totals and only got 1.53 Gallons total into the tank. I did the math like 3 times and... 47.7 MPG was still the answer all 3 times. That's VW TDI Mileage in a big car with a 3.0l 145HP vs. the tiny Jetta and 1.8 4 cylinder VW diesel!

I'm obviously going to do more experimenting with this. A one time result isn't conclusive on any experiment. But... it's pretty convincing to me that driving slow pays off in fuel consumption. I was shocked at by how much!

The errors that inevitably occur on a very short fillup can wildly affect the data.

Do the same test with a 20 gallon fill and I'm quite sure you'll find that the result will drop to 36 mpg or so............still quite good.

Interesting.....BUT......
 

presently I can afford to drive in my normal manner......usual average, timely, flowing with traffic starts, and 82 - 84 MPH on the interstate (my 300D is at some sort of natural harmonically smooth, sweet spot).......

Good info if I ever need to save more money on fuel......but I am just using my 190D, 5 speed, more often these days....and get about 40 MPG with it....

SB

Very True Brian,
Of course this is one run, I didn't check the data for the trip back home, other things. But I do know the mileage (point A to point B Gas station Mapquested) and I used a slope to fill the tank as full as I could get it. But of course it was a short trip. I'm going to keep doing this for a while.

I can't imagine driving an 8 hour trip at 62! Probably be more like a 10 hour trip. But it'd be interesting to see the results.

As stated above its too small of a distance/fill up to be accurate. I did the same thing before but drove even slower (50-55mph) for over 225miles....and managed to squeeze 31.3 mpg out of my 617 powered SD.....about 4-5 mpg better than usual. Its possible to boost it quite a bit if you go real slow and gentle. I've seen as high as 27.7 from ONLY city driving, I drove SO slow and hypermiled everywhere possible.....it almost drove me insane though. Two weeks of driving around never revving up more than 1500-1600rpm, coasting most of the time....always in 4th gear.....just plundering along....it was mental torture, but I got 3-4 more mpg than I usually get.


When you begin a trip like that your fuel in your tank is cold.....after the fuel circulates for 1.5 hours while driving it warms the fuel in the tank, which causes it to expand, possibly enough that you could only fit 1.53 gallons in, had you parked, waited half a day or a day, and then filled it, I bet you'd have fit in over 2 gallons since the fuel would have cooled back down in the tank. It was very likely the expansion of the fuel in the mostly full fuel tank that reduced the amount you could put in.

That's a good point. I am going to try it again on a longer trip. I can't stand the idea of making a several hundred mile trip 1-2 hours longer but at the same time I'm interested in the results. Probably would never try it again after the torture of driving that slow for 10 hours though! (I drive from MO to WI and MO to OK several times a year since both my parents and wife's parents live out of state)

Try 2,200 miles, almost all of which between the speeds of 60 and 63....with the wagon FULL, the entire way. See photos! (Second photo is the view from the front seat looking back...)

http://i183.photobucket.com/albums/x...l/DSC04420.jpg

http://i183.photobucket.com/albums/x...l/DSC04423.jpg

BC nailed it. A very short run like that is error prone.

Filling the fuel up to the neck lip vs 1/2" from the lip can make a huge MPG difference at those low test volumes.

48mpg just isn't physically possible in a heavy 300D without extreme aerodynamic body modifications and lightening.

I guess I didn't say what I meant there. I filled the neck until it spilled out. I actually took the gas pump nozzle and pulled it out and filled until I saw it to the top. Waited for foam to go down and did it again.

And there's still the temperature expansion of the fuel too.....

Plus angle of the ground and weight distribution in the car.

I don't think that 48mpg is impossible. I would guess that the speed to achieve it is closer to 30-35mph though. Been there, done the calculations, just need to keep the RPMs above the stall-speed of the TC and the engine in an efficient range. Aerodynamic drag is the largest consumer of fuel/power at highway speeds, and the increase of drag vs speed is logrithmic. I used to run fuel totalizer equipment on my cars and read direct fuel flow vs distance for MPG information, you can often double the 70mph fuel mileage by driving slow enough.

At 60mph though, I'd be willing to estimate a well-tuned '87 300D automatic with normal load and good tires with max. pressure will return closer to 36mpg. Mine gets 27-28 in town (driven "spirited") and 32.5 last time I did a pseudo-highway tank at 75mph.

Not to counter this point but if you look at my first post, I angled the car. I purposefully looked for a sloped spot to fill the car at. This way the filler neck was the highest point.

But of course there are other factors as PawoSD mentioned (expansion) probably more that could be fudged on a short trip. But I worked hard to eliminate what I could.

And this is a sample of one test. I'll be driving slow for a while!;) It'll be nice to see more results. Also be nice to see what others here are finding with 300d's and slower driving. Of course we all drive differently but results should be somewhat similar.

Next time you drive check this out. Drive 70-75 and notice where your foot is on the pedal relative to ZERO then back it down to 55-60 and check it out again, you'll be surprised how much closer to zero the pedal is. On my car if the pedal were a volume knob, it's about half way or say 5 out of 10 at 70MPH and it's more like 1 or 2 on the dial at 60MPH (just barely touching the pedal). There's a huge difference. That alone was pretty interesting to me. Of course pedal position can be relative but I'd assume if the linkage is properly set it should be somewhat uniform from my car to yours.

True that the pedal position changes significantly, but the lower fuel flow rate / gallons per hour is also creating a lower miles per hour so it doesn't all go to MPG increase.