Confused about 300SD MPG - PeachParts Mercedes-Benz Forum

braverichard · #1 11-30-2005, 12:00 AM

One day I decided to conduct a simple experiment. I've read numerous places that the most efficient speed for most car engines is 55mph. Anything more or less and you lose some mpg and considering that my car was made back when the national speed limit here was 55mph, the logic sounded reasonable as applied to the 1984 300SD. So I decided to drive approximately the same number of miles on the city and highway for 2 sets of 2 fuel tank fills with one difference between the sets: two tank fills will be done with highway mph of 55mph, and the other two at 65mph. I tried to keep all the miles covered the same by going only to specific places in the 300SD (166K miles) and then driving other cars when I needed to drive more.

The results are right below:

Tanks Fills Set #1
-305 miles @ 55mph, 38 miles in city driving max. speed 45mph. Total miles covered: 342. Average MPG: 24.84
-305 miles @ 55mph, 41 miles in city driving max. speed 45mph. Total miles covered: 350. Average MPG: 25.11
Overall Average MPG: 24.975

Tank Fills Set #2
-305 miles @ 65mph, 36 miles in city driving max. speed 45mph. Total miles covered: 341. Average MPG: 24.1
-305 miles @ 65mph, 44 miles in city driving max. speed 45mph. Total miles covered: 343. Average MPG: 22.9
Overall Average MPG: 23.5

I expected the 55mph results to be drastically better than the 65mph results considering the above facts, but as the results show, that wasn't the case. The city driving at all times were petty much similar, so I know that the city mpgs much not have varied by much. So what gives? I'm confused here.

No more 55mph for me. Apparently doing 10mph slower on the highway doesn't quite save enough diesel to make up for the sacrifice of driving slower.

Pete Burton · #2 11-30-2005, 12:09 AM

I think you need to do it more than once. I do know that moderate speeds and acceleration WILL give you better mileage. I once logged just over 500 miles on 1 tank of fuel some years ago on a trip to NH in my SD, when the engine was a bit worn out even. What helped a lot is on the way up there was a nasty snowstorm and I drove about 50 like there was a balloon between my foot and the petals. Mostly just survival.

#3 11-30-2005, 12:13 AM

Your results sound reasonable. You kept the city driving the same and reduced the highway driving speed by 10 mph, resulting in a 6% increase in mileage. I wouldn't expect the difference to be much more than that. I suspect that if you compared 85 mph to 55 mph the results would be more dramatic.

However, all you mileage figures seem a little low to me. Have your valves been adjusted within 15K miles? Is your ALDA adjustment reasonable? Are your injectors in good shape (no much nailing sound)?

81Wagon · #4 11-30-2005, 12:28 AM

Oddly enough I've never found my speed to have much effect on my mileage. It makes no sense whatsoever, I'll admit. I usually drive 70-75 MPH on the highway and get around 23-24 mpg in my wagon, and haven't noticed any difference when I drive slower.

I got the best mileage ever in my wagon just the other day going south to San Diego and back. Spent about 1.5 hours going an average of 17 mph in stop-and-go LA traffic, spent the rest of the time driving 75-90 mph, had the A/C running half the time and the windows down the other half... 26 mpg that weekend.

I can't explain it.

braverichard · #5 11-30-2005, 01:23 AM

Quote:

Originally Posted by Craig

Your results sound reasonable. You kept the city driving the same and reduced the highway driving speed by 10 mph, resulting in a 6% increase in mileage. I wouldn't expect the difference to be much more than that. I suspect that if you compared 85 mph to 55 mph the results would be more dramatic.

However, all you mileage figures seem a little low to me. Have your valves been adjusted within 15K miles? Is your ALDA adjustment reasonable? Are your injectors in good shape (no much nailing sound)?

Well like I said before, 6% isn't worth the slower pace to me. So I'll keep driving fast.

My injectors are all superb, valve adjustment was only done today. I slipped on my strict maintenance routine on that one, since I let the car go for 18K miles between adjustments. As for my ALDA, I'm getting the feeling the car is running rich, or my Blue 300SD (251K miles) is running too lean. Here's my reasoning: my Blue 300SD gets between 24 and 27 mpg in mixed driving all the time, has plentiful power for getting up to speed but is dead slow in acceleration below 40 mph. The 300SD that was the subject of this experiment has 167K miles, also gets up to the higher speeds, is much louder than the other one at all times (idle, city speeds, highway speeds), is much faster and always gets between 21 and 24 mpg. In fact, I once drove both of them back to back on the same short trip and the faster 300SD makes itself look like an AMG devil when compared to the other one. I haven't yet consulted any MB pros for explanations, but hey, what do you think? Is the ALDA to blame for all this? Sounds like it to me - slow 300SD gets less fuel, accelerates slower, gets better MPG as it wastes less fuel, etc?

blueranger · #6 11-30-2005, 01:48 AM

driving speed has noting to do with gas mileage
in a modern car....

back in the 70's yes 55 saved gas but over drive was
invented so the rpms would actually reduce when it clicked from
3 to 4 or over drive and you could be driving 70 with the same rpms
and 55...

braverichard · #7 11-30-2005, 01:52 AM

At a certain point, more power would be needed to fight wind and so mpg will decline with more speed. But indeed, with the newer cars with overdrive, 55mph won't yield a noticeably better mpg than 70mph. However, who is talking about modern cars here? We're talking about my 22 year old 300SD!

pawoSD · #8 11-30-2005, 02:23 AM

I hardly ever see a difference in my mpg's, its always between 22 and 23ish, If I am steady 80 on the freeway I get 23, steady 65 I have achieved 26, and at steady 55-60 I have gotten 29, haven't seen any numbers like that in a long while though. Lately its all been 22....but I usually drive it hard, so I don't expect them to be high #s anyways....

blueranger · #9 11-30-2005, 02:23 AM

the gas crisis happend
under carter.. 1976 and
by 1982 or 1983 we had overdrive...

so our mb's are modern compared to the 70's

braverichard · #10 11-30-2005, 02:52 AM

The W126 300SD does not have overdrive.

MS Fowler · #11 11-30-2005, 05:07 AM

Engine speed has less to do with fuel mileage than you might think. Engine load has more to do with it. The savings at lower speeds is related more to the reduction of load. Wind resistance is not linear; its geometric. The resistance due to speed thru the air squares as the speed doubles. In other words if you double the speed, you increase the wind resistance by a factor of four; a tripling of speed results in nine times the wind resistance.

MS Fowler · #12 11-30-2005, 05:11 AM

Quote:

Originally Posted by braverichard

Well like I said before, 6% isn't worth the slower pace to me. So I'll keep driving fast.

As for my ALDA, I'm getting the feeling the car is running rich, or my Blue 300SD (251K miles) is running too lean.

Technically, diesels are incapable of running either rich or lean. They are unthrottled, and will run at whatever speed the amount of fuel will allow. If you are overfueling for the given load, the engine will speed up, if insufficient fuel is metered, the engine will slow down. Diesels run at an equilibrium between fuel and load.

t walgamuth · #13 11-30-2005, 07:54 AM

and others.

if you tested the difference b etween 75 and 85 because of the squaring effect you would find a greater difference. iw ould bet double. the difference. not double the mileage.

tom w

braverichard · #14 11-30-2005, 09:15 AM

Quote:

Originally Posted by MS Fowler

Technically, diesels are incapable of running either rich or lean. They are unthrottled, and will run at whatever speed the amount of fuel will allow. If you are overfueling for the given load, the engine will speed up, if insufficient fuel is metered, the engine will slow down. Diesels run at an equilibrium between fuel and load.

Yes, I am quite aware of that (same reason, lack of throttle, why diesels produce no vaccum at idle, at least with all things in normal condition on the engine). However that's all technically speaking. What does the ALDA do then? Doesn't it control fuel flow to the injection pump? Or at least play some part in it?

#15 11-30-2005, 09:23 AM

Quote:

Originally Posted by braverichard

Yes, I am quite aware of that (same reason, lack of throttle, why diesels produce no vaccum at idle, at least with all things in normal condition on the engine). However that's all technically speaking. What does the ALDA do then? Doesn't it control fuel flow to the injection pump? Or at least play some part in it?

Aside from the semantics, the ALDA controls, in part, the amount of fuel that is supplied to the engine. Turning the ALDA "up" in the CCW direction will increase the amount of fuel resulting in more power, as well as potentially more smoke and lower mileage if you go to far. The change in power is most noticeable below 2000 rpm.