This is my 3rd tank of fuel and I am averaging about 26 MPG. Virtually all of my driving is expressway at about 75MPH. I was expecting more like 30 MPG. Ironically, my MPG seems to be better in city driving than expressway driving. This engine is only 67HP. My neighbor gets about 25MPG in his 200+ HP Acura 3.2TL.

I think the problem is with the gearing. I have a 4spd manual and 3.69 rear. In 4th gear, the RPM's seem to always equal 1/2 the speedometer speed times 100. So, at 70MPH my RPM's are 3500 and at 80 MPH, my RPM's are 4000. A diesel engine turning 4000 RPM is not a good sound.

I think a 5th gear would do SO MUCH to improve fuel economy. I am at a loss for how to improve the MPG on this 240D. Swap axle from a 300D? Maybe install larger tires?

Does anyone share my disappointing results?

in my experience your expectations were a little high. back off to 65 and 30 is probably possible. in all my 240s i seldom ever reached 30 mpg. and around town it was like clockwork...20 mpg.

yeah a fifth gear would improve it about 20%. oversize tires will do the same thing but 20% would be hard to reach. and oversize tires will ride rougher and upset the handling and steering by kicking up the rear end.

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

You might also try adding a cetane booster like Power Service to each tank of fuel.

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Never a dull moment at Berry Hill Farm.

There is a reason that you can't successfully install a taller axle in the 240. With only 67hp, you'll take an hour to get to 70 mph if you had a 2.88 in it.

Furthermore, you'll never get anywhere near 20% increase in fuel economy. For those folks that reduced the ratio on the 300D from 3.07 to 2.47, they could document not more than 10%. I'd suspect that you would get something less than 10% as a maximum benefit with a 2.88. This comes at a significant hit to driveability. The slightest upgrade on the highway and you'd be downshifting to third with a resulting engine speed of 4400 rpm.........there goes any fuel economy benefit.

You can't compare apples and oranges. Your 240D was getting 20-30 MPG when other cars its size were getting about half that. Today's gassers will typically beat a 240D's performance and economy, those are not necessarily its strengths today. The 240D was obsoleted by the more powerful 300 Turbo Diesel because the 240D only had 67HP. The turbo gives you more power and virtually the same economy, if not better.

Longevity, ease of repair and reliability are what you need to concentrate on with your car. Your neighbor's Accura will likely be recycled twice into new cars before it reaches the age of your 240D!

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz

I get 32MPG from my 1977 240D on hignway doing 60-65. The car has new motor & is standard. Mine will do 75 with no problems but I never go that fast, do not like the sound of that little motor doing 75, sounds like it is about to explode.
Expect to get a louzy fuel economy it you do 75, the little motor simply does not have the power to fight the wind.

Vahe
240D 1977/350K

I get about 31 mpg on my all-around driving. Typically I don't run much over 70 mph on the interstate. And I am pretty sure I was getting better mileage at 65 mph. The car will go 80 plus with no problem, but when you take it past 65 mph the fuel mileage seems to go down exponentially.

These cars were built and geared for the American market, when the national speed limit (yes, even on Interstate highways) was 55 mph.

At least you don't have an automatic.

Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles

I have driven this 240D at 95MPH. I was actually surprised how quickly it would reach that speed. The engine sounded like it was going to explode though - turning at 4750 RPM. I think you are all correct. The only way to achieve better fuel economy is just to s l o w d o w n.

Does anyone know the "sweet spot" with the 240D? By that I mean, the best speed / engine RPM when the optimal efficiency is reached. For example, 50 MPH at 2,500RPM, or maybe 40MPH at 2,000 RPM.

There is no magic "sweet spot". The friction across the body decreases with speed. The slower you go, the less fuel is required to push the vehicle through the air. So, if you only consider wind resistance, then 1 mph is the sweet spot.

But, the engine burns fuel whether it's moving or just sitting there doing nothing. So, if you drive 40 mph, the engine has to run 25% longer time than if you drive 50 mph to get to your destination. This burns 25% more fuel for the simple act of running the engine.

The best economy is where the two curves cross each other.......where the fuel consumption over a given time due to the wind resistance crosses the fuel consumption over a given time due to running the engine under no load.

You're probably correct in that it's usually between 40 and 50 mph. Proabably a moot point.......who's going to drive that slow?

As an aside, I drive the Dodge B-250 with about 500 lb. in the back at 55 mph......never faster. I routinely get 16 mpg in the warmer months........nobody matches that number with a full sized 3/4 ton van. I slowed it to 50 mph for one tank.......when gas went over $3.00........and got the best fuel economy ever.........17.25 mpg.

I have had my 1982 240D for about two years. I, too, was very disappointed in the MPG. I don't drive it all of the time, but when diesel just dropped back down to around the price of regular unleaded, I got it back out.

I have been looking at a VW TDI (not as a replacement) and was reading on a TDI Forum about someone well known for his high MPG in a VW TDI. He replied that it has a lot to do with the way you drive, which of course makes a lot of sense.

I have started shifting a lot earlier, no more "Italian Tuneups" on a daily basis. I try to run just above the shift point for the next lowest gear. i.e. I shift into third around 25 mph and fourth around 40 mph. I have been trying to keep it around 65 on the Interstate. I drive in the Ozark Mountains, and some flat land.

It has jumped up to just over 30 mpg for the last two tanks. My goal is to top 100 miles before the gas gauge gets to 3/4, over 200 miles before 1/2 tank, etc. I always try to fill it up to the top, for more accurate calculations. It can be foamy, and look full, but if you wait, it may burp out some air, and take a little more fuel. I divide the gallons to fill the tank, into the miles driven to come up with the miles per gallon.

I have also used the Cetane Booster, but not on a regular basis, usually just before I know that it will probably be setting up for a while.

For straight cost per mile, you are doing a lot better in your Mercedes 240D than your friend is in his Acura, when you factor in the purchase price, cost of insurance, etcetera.

Personally, I think my 240D feels happy at about 65 mph. I think that's about 3500 rpm, and it just gets too buzzy at higher rpm's. AFAIK, 3500 rpm is about the peak torque for this engine. The 240D feels the same at 65 as the 300D does at 80, which is consistent with the 20% (about) difference in gearing. Driving the 240D at 75 feels like driving the 300D at 90 or 95 mph, it will do it, but it's not happy to stay there very long.

Peak torque is much lower. The 300 has a peak torque at 2400 rpm, which I always have trouble believing, but, Randy has charts to prove it.

the 20 % figure was my estimate for an 5 speed od tranny. larry has one in his 240 and that is not far off what he gets. 30 to 35 mpg. nobody comes close in a four speed 240. you wont top 30 unless you drive very very slow.

i tried not to go over 72 in my 240s.

they are the champion for cheap operation. you just cant get in a hurry. they are pretty engaging to drive with a stick but never fast. once used to it you get in a rythem and it feels fine.

i wouldnt advocate changing to a higher gear in a 240, but a five speed would be the best.

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Wow, that sounds low, but it is what it is. Maybe 3500 rpm is closer to peak HP than torque?

The partial answer like Tom mentions is gettting a five speed in there. Almost any other modification is going to reduce acceleration. Thats just intollerable to even comprehend. I think the quieter engine at speed alone is worth the extreme effort to find one. Will probably save a little fuel as well but that car cries for an overdrive. If I ever locate one it will almost be a miracle. Wonder if they are almost a surplus item in europe compared to here? Unless the transmission is longer it would be a direct bolt in transmission change as well I believe. Anyone know if it is longer?