hello all i bought a 1985 300d about three weeks ago. I just did a 300 mile road trip (all interstate and mostley flat just two mountains). I only averaged 23mpg and i was told that these cars are capable lower 30's. Anyone have any ideas what i can do to improve it?

 

That is within the range of the factory reported MPG estimate of 19-23.

Those who claim low 30s are mistaken.

http://www.fueleconomy.gov/FEG/noframes/337.shtml

 

I drove many many miles in those 5 cylinder turbodiesels back when they were new. The best I ever got on the highway was 27 with 23-24 being most common.

Now that these engines are 20+ years old there is a high likelihood that mileage is diminished as a result of wear, some of which you can fix and some of which you can't.

If the flow rate in the pump gets off balance you will waste some fuel by running those cylinders rich. You will also waste fuel from unevenly clogged injectors.

You will lose a bit of fuel economy if the valve adjustment is off

Finally you can get a small improvement with light synthetic fluids in the engine and driveline. The friction reduction makes a difference. Get the tires to max pressure too.

 

You can get near 30, however, you will be going about 55mph.

 

FI,
Did not the 84 & 85 have a better rear ratio, resulting in better fuel economy? I would expect for them to approach/achieve high 20's with cruise 65-68.

I can get 23-25 (I think, I need to clock it again) but I keep it 64-65.

Obviously the winter blend will degrade the mileage.

Gary

PS, the real liars are on Ebay.... Like you all have not experienced this.....

 

Yes the new fuels and winter fuels hurt the MPG'sI have a 87 300D and 30mpg isn't even a dream of achieving. I've run sweet crude heavyely filtered top grade fuel at the pump tried additives. A mixture of crude diesel and lucas additive got 28 MPG over a 2500 mile test but I got tired of saying below 65 mph 26 was my best running 70 75

 

Just took the 300sd out. Went to Tampa from Orlando. At 60mph I used 8.3 gallons for 220 miles. It was 60 degrees out and the car has 38,000 miles on it. No I didn't forget a zero.

 

OK and your figures = 26.5mpg If you want to get a better average of mpg it need to cover a few fill ups. Depending on how the car is sitting and who's pump nozzel to whether you got it as full as before. Putting in a mear 1/2 gallon more changes it from 26.5 to 25mpg a 1/2 gallon less makes it 28.2mpg. So a monthly average is a lot better. I'am never one to get any of the MPG's some people cliam but I also will step on it to go around some one before I go 5mph slower to follow them. All the more miles I put on a year 10k 30mpg over 20 would save me 167 gallon I'll pay for it and run as fast as I want. Why own a Benz then try to be a obstacel on the highway for most others, you own a classic run it, or buy a hugo!

 

I have a 190D and I track my mpg's consistently in a little book. I have made a long distance trip to NJ and back and average 35 mpg, and I was doing well over 65 mph. I went to MO for Thanksgiving and my average then was 30± (had fuel filter issues & had to change the filters). I always try to get my fuel at the same place where my log book shows that I got the best mileage. My weekly average is in the low 30's due to "city" driving etc. I guess compared to a 300 my car is very little and doesn't weigh as much. Not to be a dud, but wouldn't the size and weight of the car make a difference? Perhaps that contributes to the lower mileage on the bigger sized cars.

 

Hmm, I hope that is a good thing. It is an '89 4 dr. It only had 140K when I got it. The body is in great shape. Exterior wise, it was kept in good condition, however, the owners obviously didn't understand what routine/basic maintenance meant. The oil looked like it hadn't been changed since the car drove off the assembly line. It was low on transmission fluid, low tires, glow plug issues, undersized battery etc. I've been having to replace a lot of little things but with each little repair the engine runs better and the ride get smoother. All that is left now is to ditch these low profile tires and get a nice standard set.