MPG vs Speed - Page 4 - PeachParts Mercedes-Benz Forum

t walgamuth · #46 04-10-2013, 11:10 PM

Quote:

Originally Posted by Brian Carlton

It is possible that the carburated engine is running on the idle circuit at 55 mph with very poor distribution.

At 75mph, the main venturis are running and the distribution is significantly better and the engine is more efficient in terms of lb./hp/hr.

However, this does not imply that I believe that such a vehicle actually overcomes the additional 1.5X horsepower requirement at the higher speed.

The op has a fuel injected engine.

tbomachines · #47 04-10-2013, 11:12 PM

Quote:

Originally Posted by t walgamuth

You clearly don't understand the physics if you think that....sorry.

How so? Take engine RPM and fuel efficiency as constant. Mess with gearing, aerodynamics and rolling resistance and its pretty much infinite minus bearing friction and heat loss.

tbomachines · #48 04-10-2013, 11:19 PM

Quote:

Originally Posted by Brian Carlton

You know better than to make a totally factually incorrect statement such as the above.

The reason is simple. It takes 1.5X horsepower and more horsepower requires more fuel.

Gear it down far enough? I am not talking real world.

Edit: ok I see the gripe, I omitted the idea that I am talking about different gear ratios. Ideally you can gear each to be as theoretically efficient as the fuel and engine allows at whatever speed you desire, again in a hypothetical situation (basically a vacuum with zero resistance tires). My mistake.

Edit 2: well, I realize that doesn't really add anything to this conversation anyways since ideal situations don't really exist in practice. So um...have a good night!

Brian Carlton · #49 04-10-2013, 11:26 PM

Quote:

Originally Posted by t walgamuth

The op has a fuel injected engine.

Well...........I tried..........

t walgamuth · #50 04-10-2013, 11:52 PM

The one thing he said that can be true with no mistakes is running in a pack of big trucks.

I used to have a very low powered car, a 1980 Datsun b210 wagon with a 1300 cc carbureted engine and a five speed. I Made the mistake of putting on one size larger tires and that car was just on the knife edge of being able to maintain a cruising speed in fifth gear. It would run about 65 flat out on its own in fifth, faster in fourth. (it always got over 30 mpg highway).

So one day we were traveling from Texas toward NOL on that interstate that runs along in the swamp dead level, running in a fairly tight group of cars and I found that I could run 70 or 75 no problem.

But get in a pack of cars (not trucks mind you) and that datsun would run 75. I could even draft a 240d from a couple of car lengths back (and did one day all the way back to Lafayette from Indy on the interstate running well over what I could run solo.)

In a big group of cars the air is pulled along and everybody will be getting better fuel economy.

Same phenomina as the Nascar guys running faster in the race than qualifying, the pack moves the air along and everybody benefits.

So running in a big group of semis, yes, I believe it is possible to get 23 mpg in that car running 70 to 75. This effect would be magnified with semis and a car.

Or running in third gear 65 and fourth at 70 perhaps.

But not with all things being equal.

Emmerich · #51 04-11-2013, 12:53 AM

The OP math is off, to get to 1 MPG would be taking your 55 MPG and multiply by .95 until you get to 1.

The drag increases with the square of velocity, the force is non-linear. Also, different engines have different power curves. Depends on the cam and stroke and flow characteristics of the head/intake/etc.

I had Porsche Turbo that got its best MPG at 110. In top gear it was very slightly into the boost which apparently was pretty efficient.

Brian Carlton · #52 04-11-2013, 12:56 AM

Quote:

Originally Posted by Emmerich

I had Porsche Turbo that got its best MPG at 110.

This is a factually incorrect statement and cannot be supported by any kind of realistic data.

It must be dismissed.

t walgamuth · #53 04-11-2013, 06:11 AM

Quote:

Originally Posted by Brian Carlton

This is a factually incorrect statement and cannot be supported by any kind of realistic data.

It must be dismissed.

x2. How could such a thing even be established? Not even in Texas could you run a tank through at that speed.

pmckechnie · #54 04-11-2013, 10:03 AM

OK here is some data for 2013.

As you will see, the MPG ranged from 10.302 to 20.509

The last 4 lines are the trip from Stallings, NC (Charlotte, NC) to Orlando, Fl and back. The route was I-485, I-77, I-26, and I-95. From our home to I-495 is about 1 mile. We actually stayed in St. Augustine as our hub for the weekend. We started out with fuel bought on 3/23 with about 70 miles on that tank when we left. First fill up was on 4/5 in Walterboro, Sc and the MPG was 16.257 for that tank. Remember it was mixed driving. Second fillup on 4/6 was in Ormond, Fl. and was 20.509 mpg and was all interstate going 70+. Third fill up was in Elkin, Fl and was 16.188 MPG. This was mixed Interstate and local driving. Fourth fill up on 4/7 in Bowman, Sc. and was All interstate at 70+ mph and was 19.898 MPG.

Would you please plug your physic formulas in this and explain why the 2 highest MPG were when it was all interstate at 70+ MPH. I think you will have to agree that a 126 gas car getting 20MPG is unusual to begin with but this car will do it at 70+ but about the best otherwise is about 16. I really wish it was just the opposite like you are trying to tell me it should be.
The other 7 years of spreadsheets show the same results in most cases. On a trip to Maine last year (2500 miles) we got the same results. The best MPG were on the days we spent most of the time on the interstates and dropped on the tanks with mixed driving.
You can call me crazy, poor record keeper, poor math skills, impossible, or what ever you want, BUT all of this is true. I guess I could drive on the interstate with no stopping at 55 mph to see what happens but I had rather live a few years more. Driving on State roads at 55 to 60 does not show an improvement, but rather a lower MPG.
By the way, this car runs GREAT at any speed. Actual torque is a little off below 2000 RPM but not enough to call it a problem. It just starts pulling hard above 2500 to 4000 or so. I usually don't let it get much higher than that except when doing a "Tune-Up". Then the rev limiters stop it at about 5500 and it is still pulling hard up to that point.

What am I doing wrong to come out with these numbers?

Paul

Brian Carlton · #55 04-11-2013, 10:14 AM

Quote:

Originally Posted by pmckechnie

the MPG was 16.257 for that tank. Remember it was mixed driving. Second fillup on 4/6 was in Ormond, Fl. and was 20.509 mpg and was all interstate going 70+.

Would you please plug your physic formulas in this and explain why the 2 highest MPG were when it was all interstate at 70+ MPH.

The answer is quite simple.

Getting the vehicle to accelerate and then waste that energy with the brakes consumes fuel..........lots of it. This is especially true with a big V8 that consumes quite a bit of fuel under minimal load conditions.

Take the same vehicle out on the highway at 70 mph, and, despite a horsepower penalty of about 25 hp (versus tooling around town), there is no acceleration and braking loss, and the fuel economy improves.

If you were to do all city driving below 20 mph with that vehicle and compare it to your highway fuel economy, you'd find that you got less than 50% of the highway fuel economy in the city.

Smaller and lighter vehicles are going to do better in this regard because the energy to accelerate and stop them is a fraction of what is necessary for yours.

JamesDean · #56 04-11-2013, 10:29 AM

I drove my friend's newly acquired 560SEL from NYC to Ohio, all highway, with cruise control on. It achieved 21-22 MPG.

My 420SEL, when we would drive to Pittsburgh, could do 20-21 MPG.

My 190E would do 19-20 in town, but 23-25 average on the freeway.

pmckechnie · #57 04-11-2013, 11:31 AM

Brian, We go to the Outer Banks of NC every year. About 370 miles one way. We have tried different routes and some of them were 90% 50 to 60 mph and the best mileage we have seen is around 17 mpg. Actually the best we have seen on the trip regardless of route is about 19 which is about 50% interstate and the rest is state roads at lower speed. In other words, the faster the better. After studying this deeper I am convinced that the key is engine efficiency. My engine which is made up of many different engines from who knows where happens to be most efficient at 2500 rpm or higher. I believe you when you say Higher speed takes more HP, but not that higher speed takes more fuel for miles traveled. It depends on efficiency of the engine at the speed it is traveling. Every car will have a sweet spot where MPG is highest and that could be a higher or lower speed depending on gearing. I may be able to change the final drive from 2.47 to say 3.00 and get better mileage at 55 than at 70. On the other hand, I may make the final drive 2.22 and actually get better mileage at 80 than at 55. Right now I believe the engine is laboring at 2000 rpm at 55 mph and is suffering because of it. In other words, there is nothing wrong with the engine, fuel system, etc, but has a gearing problem for driving in the US.
My whole reason for starting this thread was to understand why the mileage was higher at a higher speed. I feel I know the answer and physics has nothing to do with it in this case. Laws are made to be broken. I am sure you know that before physics took aerodynamics in to account, the bumble bee could not fly. The wing area to weight ratio made it impossible. Poor bumble bees had to walk every where they went.

Paul

wildhorsemfg · #58 04-11-2013, 11:51 AM

the only was to do a acurate measurement if to have 2 tanks ,
first main one for accelerating then you switch to tank b
tank b has a flow meter on it ,
accell , switch to b , drive xxx miles , switch back , record usage/ miles
repeat with diff speeds 100 / 70 / 55 mph

t walgamuth · #59 04-11-2013, 01:02 PM

Quote:

Originally Posted by pmckechnie

Brian, We go to the Outer Banks of NC every year. About 370 miles one way. We have tried different routes and some of them were 90% 50 to 60 mph and the best mileage we have seen is around 17 mpg. Actually the best we have seen on the trip regardless of route is about 19 which is about 50% interstate and the rest is state roads at lower speed. In other words, the faster the better. After studying this deeper I am convinced that the key is engine efficiency. My engine which is made up of many different engines from who knows where happens to be most efficient at 2500 rpm or higher. I believe you when you say Higher speed takes more HP, but not that higher speed takes more fuel for miles traveled. It depends on efficiency of the engine at the speed it is traveling. Every car will have a sweet spot where MPG is highest and that could be a higher or lower speed depending on gearing. I may be able to change the final drive from 2.47 to say 3.00 and get better mileage at 55 than at 70. On the other hand, I may make the final drive 2.22 and actually get better mileage at 80 than at 55. Right now I believe the engine is laboring at 2000 rpm at 55 mph and is suffering because of it. In other words, there is nothing wrong with the engine, fuel system, etc, but has a gearing problem for driving in the US.
My whole reason for starting this thread was to understand why the mileage was higher at a higher speed. I feel I know the answer and physics has nothing to do with it in this case. Laws are made to be broken. I am sure you know that before physics took aerodynamics in to account, the bumble bee could not fly. The wing area to weight ratio made it impossible. Poor bumble bees had to walk every where they went.

Paul

When you are on the roads driving 55 to sixty you are also stopping and starting regularly. the interstate offers constant speed and ideal conditions for good economy.

Not rocket science.

JamesDean · #60 04-11-2013, 01:04 PM

Quote:

Originally Posted by t walgamuth

When you are on the roads driving 55 to sixty you are also stopping and starting regularly. the interstate offers constant speed and ideal conditions for good economy.

Not rocket science.

Add in steady cruise control and you will see your numbers become more consistent. I saw my 300D's numbers steady out after I left it up to the cruise control.