MPG vs Speed - Page 2 - PeachParts Mercedes-Benz Forum

BobK · #16 04-10-2013, 08:58 AM

What rpm at 55? What rpm at 75? Does it have a cat? True dual exhaust all the way back? I assume fuel injected. I don't doubt the numbers you list. Could just be that you are too low on the torque curve at 55. Engines can be tuned for max efficiency at a specific rpm. They do this with stationary engines like generators that need to run for a loonngg time at a very specific rpm with very little variation.

t walgamuth · #17 04-10-2013, 09:02 AM

Quote:

Originally Posted by BobK

What rpm at 55? What rpm at 75? Does it have a cat? True dual exhaust all the way back? I assume fuel injected. I don't doubt the numbers you list. Could just be that you are too low on the torque curve at 55. Engines can be tuned for max efficiency at a specific rpm. They do this with stationary engines like generators that need to run for a loonngg time at a very specific rpm with very little variation.

no, sorry. nothing there will cover a 20 mph difference in speed.

pmckechnie · #18 04-10-2013, 09:37 AM

BobK The rpm at 75 is right at 2500. Don't rem what it is at 55. I think I need to say that the 55 really has nothing to do with my question. I just know I get better mileage at 75 then any speed. Car has single exhaust, No cats, CIS injection, No computer controlled mixture. I have logged all fuel and mileage for the last 8 years and can see that the best mileage is when on long interstate trips.

Well, time to close this thread. I've been told in so many words that I don't know what I am talking about yet my numbers tell me otherwise. I believe my numbers and others can believe what ever they want.

But thank you BobK. I think you have hit on a possibility of efficiency at an RPM. I guess my car just likes 2500 rpm.

Paul

t walgamuth · #19 04-10-2013, 10:09 AM

I had an 84 500 SEC and could never top 20 mpg at 65. Faster speeds got much less mpg, though it was such a nice ride that driving 80 mph was worth the price in fuel.

Running 80 to 85 would drop it down to about 16.

I drove that car about 50,000 miles and recorded every fill up and the mpg each time.

t walgamuth · #20 04-10-2013, 10:11 AM

Quote:

Originally Posted by pmckechnie

BobK The rpm at 75 is right at 2500. Don't rem what it is at 55. I think I need to say that the 55 really has nothing to do with my question. I just know I get better mileage at 75 then any speed. Car has single exhaust, No cats, CIS injection, No computer controlled mixture. I have logged all fuel and mileage for the last 8 years and can see that the best mileage is when on long interstate trips.

Well, time to close this thread. I've been told in so many words that I don't know what I am talking about yet my numbers tell me otherwise. I believe my numbers and others can believe what ever they want.

But thank you BobK. I think you have hit on a possibility of efficiency at an RPM. I guess my car just likes 2500 rpm.

Paul

I agree completely with the part highlighted. And it is true if compared to driving around town stop and go, but not if compared to running 55 or 60 mph highway.

t walgamuth · #21 04-10-2013, 10:14 AM

Quote:

Originally Posted by pmckechnie

Any one want to comment on this. It is generally stated that for every 5 mph over 55 will reduce your MPG by 5%. I just got home from the central Florida GTG (Thanks TheDon) in our 500SEL. On one segment of the trip home I had a moving average of 72 mph and got 19.9 mpg. If I loose 5% for each 5 mph then I should be able to get 17% better at 55, or 19.9 + 17% or 23.3 mpg. Well after 8 years of monitoring the mileage on this car, it ain't gonna happen. Well I did get 23 mpg one trip to Florida about 6 years ago. I was running with a pack of 18 wheelers early one morning. I wasn't drafting or anything like that, but just staying in the pack. Speed ranged from 80 to 100 for over 200 miles.
Now why does this car get better mileage at higher speed? I just calculated the mpg of a 126 going 140mph on the autobahn would give it a MPG of about 1, or 26 miles per tank at a 5%decrease per 5 mph. When they finish the bridge to Europe I am going over there and find out the truth.
I believe the Germans engineered these cars to go fast but the US government doesn't want us to know that. Can anyone say for sure what the mileage is for a 126 at say 130 mph on the autobahn?

OK let me have it.
Paul

PS all data came from gas tickets, speed and distance checked by gps and all calculations done on a spread sheet. So don't say I can't calculate, I didn't have to.

If running in a pack of 18 wheelers you could probably have put it in neutral and coasted (exaggeration for comedic effect). 23 mpg is quite possible in that circumstance, but has no relationship to real world economy running solo.

BobK · #22 04-10-2013, 10:47 AM

If you are doing 2500 rpm at 75 mph (wow, that's tall gearing) then you would be doing a little over 1800 at 55. Quick check shows versions of that engine to have a torque peak between 2500 and 3750 rpm. Possible you were running too low a speed and basically lugging the engine at 55. If you were running close to the torque peak at 75 then that would make more sense to me.
Tom may disagree and that's ok. We all have our own experiences and opinions. Generally I get better economy at lower speeds too. I have had exceptions though. My Datsun was one and my '86 SDL was another. It seemed to like 70-85 better than 60-70. Surprised me too as diesels generally are real rpm sensitive for fuel economy, but other factors can play in there too.
Do you know what axle ratio you have. It should be stamped on a flat surface or a tag on the back surface of the differential housing, usually near the bottom. Hope I remember to look in my data book tonight. This is starting to get interesting.
BTW, Tom is a good guy with a lot of knowledge and I take his information to be well worth considering even if I disagree with it.

cjlipps · #23 04-10-2013, 11:00 AM

I'll agree that generally an engine is most efficient at its torque peak. However, with modern engine management systems, the torque curve is relatively flat so it would seem that fuel economy would drop exponentially once you're over about 50 with wind resistance being the primary parasite.
I have had cars in the past that did better at 65 than at 55 but everything I've owned lately (and there have been quite a few) suffers over 65.
And just for reference, the numerous Buick/Pontiac/Olds iterations I've had with the 3800 turn about 2000 rpm at 70 and the two GM 5.3s I have now (Avalanche and Yukon XL) turn about the same.

t walgamuth · #24 04-10-2013, 11:20 AM

his 500 should have the 247 diff like mine did.

Euro versions had 224 and burned premium and had a lot more power.

BAVBMW · #25 04-10-2013, 11:25 AM

Get better with speed? Not terribly common, though sweet spots do exist for every car, and it just might happen that your sweet spot is at higher speed. With tall gearing, lugging/loafing around at 55 might not be as good as a sweet spot at say, 70.

Now our little highway car, with 2.0 liters and a six speed manual, mileage does go down as speed goes up, but, not by much (within the realm of our highway trips). Using GPS, our typical trip has a moving average of about 72mph, in reality this means ~90mph on the open road, and then brought down by congestion and/or surface streets. When we do this trip in this manner, we average ~31.5mpg. Same trip at slower speeds, moving average ~55mph, highway cruising ~70mph, results in mileage of ~33mpg. So while yes, the mileage is higher at the lower speed, we opt for the reduced drivetime.

I had another car years ago that was similar. I went on a road trip and for the first time in my life broke 30mpg, I thought this was amazing, and considered I was driving plenty fast, wondered how good I could do if I drove the speed limit instead. So the next weekend, I repeated the trip. At the speed limit. I got 1.5 mpg better. I was disappointed.

MV

pmckechnie · #26 04-10-2013, 11:38 AM

Bob I believe it is a 2.47 or something like that. Remember, these cars have a 4 speed trans but is not overdrive. cjlipps mentioned Buick 3800 running 2000 at 70, but that is an overdrive trans. We had several customers that had 3800 Olds and Buicks that would get over 30 mpg.
Other cars I have had also got lower mpg at high speeds. This car is different, and the only one I have ever had like it. I can fell that the torque comes on at around 2500 and pulls hard to about 4000, so It must be at the bottom of the torque peak at 75 mph (2500 rpm). Which is good and bad. Good if on the interstate, bad if on state roads at 55 mph. Maybe 3rd gear if lower than 60. I have tried to explain "the sweet spot" of an engine. Every engine has one and it may be different on the exact same engines. I understand frontal area and CD but it comes down to how the air goes around, under, over the body. If this car was designed to have a max sustained speed of 145 mph then the engineers took that into account when they built the car. I think they did a good job. I shouldn't mention this but this car really smooths out at around 120. It is still responsive at that speed but I am sure the mpg is down a tiny bit. Like I said before, when they finish the bridge over the big pond I am going to Germany and check the mileage at 140mph. Ha HA.

Paul

jcyuhn · #27 04-10-2013, 11:55 AM

Another reason the 5% rule doesn't apply is because the Brake Specific Fuel Consumption (BSFC) of an internal combustion engine varies across its operating RPM and load ranges. BSFC is the quantity of fuel an engine needs to burn to produce a given amount of power. Basically the efficiency of the engine at converting the latent chemical energy in gasoline into mechnical work varies depending on RPM and throttle position. Here's a link to the best explanation of BSFC I could quickly google up: AutoSpeed - Brake Specific Fuel Consumption

Given the data you have, it's possible the engine is in a more efficient 'island' at 70 than 55. However given the significant extra energy required to attain 70 vs. 55 I find it difficult to believe the car can delivery better fuel economy. The engine would have to climb a very steep BSFC curve from the lower to the higher speed.

pmckechnie · #28 04-10-2013, 12:21 PM

Tom, let me tell you a little about the engine and maybe you can shed some light on this subject. The bottom end is the original 84 500 that came in the car. The heads, cams intake and fuel system came from a engine that I was told came from a car out of Mexico. Who knows for sure where the car came from. It is a low compression engine I believe because you can not make it spark knock with timing set anywhere from TDC to 15 deg BTDC on regular gas. I did convert to an ICV system as the thing that is temperature operated for fast idle when cold never worked. The fuel system does not have a frequency valve. I switched it to a system from a 380 using the original fuel computer but never got it to run right so I put the 500 fuel distributor back on with out the frequency valve. It runs great. Could this combination explain the higher mileage at higher speed?
This car was headed to the scrap yard about 12 years ago when I rescued it and made it into a wonderful car again. Best $500 I ever spent.

Paul

Pooka · #29 04-10-2013, 12:31 PM

In 1974 I drove a 1972 Chevy Monte Carlo from Dallas to Austin which is about 200 miles. This is a drive on I-35 which is pretty much a north-south route.

I drove there bucking a 45 mph wind. I got 4 mpg.

The next day I drove back. Same speeds, same road, same 45 mph wind but now at my back.

MPG was 52.

If I had just rigged up a few sails I think I could have made the trip back for free.

JamesDean · #30 04-10-2013, 12:50 PM

My 300D has been getting fairly consistent numbers. I normally set the cruise control on 70, a few times I've done 75, with marginal change in economy numbers.

I've had snow tires on since late December. When do we go back to "normal" diesel?