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I've read some alarming tales here of folks depending on the computer generated Range estimate for fuel, only to run dry on the freeway. So when I picked up my Metris Passenger Van this past spring I kept records for a few fill ups to see just how bad that Range value is.


Using a tank capacity of 18.5 gallons, and my observed town mileage of 25 mpg, I calculated the true fuel remaining after each fill up, and estimated the true range. I then subtracted that value from the displayed Range to get the "Range at Empty" value. So for 9 fill ups I saw:


72 miles
78
70
70
70
72
65
67
61
70


So the good news is that is was at least fairly consistent. But the bad news is obviously that the computer thinks I have 70 miles more range than I do. Kind of hard to understand how that value can be so far off, but that's what I've seen.
 

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Yet, you do not absolutely know how much fuel is in the tank because you are going by the spec. If you wanted to geek out completely you could drain the tank and measure that or drain it and fill to the level you are measuring at...
 

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Yet, you do not absolutely know how much fuel is in the tank because you are going by the spec. If you wanted to geek out completely you could drain the tank and measure that or drain it and fill to the level you are measuring at...

You do have a point. And in the sailing community I know of folks who have done exactly that with both their diesel and holding tanks. But then again, I do find it hard to imagine that the Metris fuel tank is close to 3 gallons larger than they claim (to give an additional 70 miles range) as I would think they would want to brag about having a larger tank.
 

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Your measurements are similar to my observations. On my first road trip, I put fuel in with 80mi range left and was shocked to put in 18.3 gal. The fuel gauge read nearly 1/4 tank with 0.2gal fuel in the tank.

The displayed fuel economy is high by some 20%, too.

Not much German Engineering to be proud of here.

It goes along with the clock that loses a minute per week.

I expect Mr Green will be here shortly...
 

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I checked with the regular van driver in my household. She regularly sees 45 miles DTE when the second fill now message comes on.
 

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But you really don't know that for sure.

Most people simply refill before 1/4 tank as a regular practice.
I love the internet.

I know the fuel gauge read just under 1/4. I know the fuel pump read 18.3 gal pumped. I know the Mercedes manual says the fuel capacity is 18.5 gal. I know that 18.5 minus 18.3 is 0.2 gallons.
 

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I love the internet.

I know the fuel gauge read just under 1/4. I know the fuel pump read 18.3 gal pumped. I know the Mercedes manual says the fuel capacity is 18.5 gal. I know that 18.5 minus 18.3 is 0.2 gallons.
It's possible you missed my point. While you know those are the numbers you read you do not know what amount of fuel is actually in your tank.

My profession is testing and measurement.
 

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It's possible you missed my point. While you know those are the numbers you read you do not know what amount of fuel is actually in your tank.

My profession is testing and measurement.
I bow to your profession and agree that I don’t know if there is 0.2 gal or 0.21 gal or similar sort of discrepancy as neither of us knows the precision Mercedes used when converting the fuel tank capacity from Metric to Freedom Units, nor do either of us know the amount of fuel in the fuel lines nor fuel filler tube. Neither of us know the amount of fuel that may actually be in the tank but not accessible to the fuel pump(s) intake(s).

But, I'd suggest, you're missing the forest for the trees.

The thread is about the poor accuracy of the mercedes metris fuel gauge. I actually do know that for a fact.
 

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But, I'd suggest, you're missing the forest for the trees.

The thread is about the poor accuracy of the mercedes metris fuel gauge. I actually do know that for a fact.
I'm not missing anything. I'm just trying to be factual since this thread is about how the tank is being measured and if the numbers are accurate or not.

We don't know if the spec is accurate and we don't know how much variance there is in the volume of the tanks. There is a sending unit that is probably mechanical. We don't know how much variance there is in the assembly, mechanics or electronics. I'm imagining the sending unit uses some resistance that can be calibrated in software, but we have no access.

So beyond being technically accurate, the best kind of accurate, you can calibrate to some relative number you know is correct for your van and your driving habits. If you believe that when the van says 70 miles remaining you only have 30 then perhaps that is always consistent. Just do that. If you want to find out... just drive your van until it runs out of gas and then use that for the baseline. If you wan to calibrate your van you can 1) adjust the mechanics in the tank 2) adjust resistance for the gauge 3) add solid objects to your tank to displace fuel... etc.

So, you say the gauge is not accurate. Well, there are many variables that contribute to the accuracy. The gauge itself might be perfectly accurate, but your sending unit is not, your wiring is bad, your are on a slope, you are smuggling something in your tank etc.
 

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Yet, you do not absolutely know how much fuel is in the tank because you are going by the spec. If you wanted to geek out completely you could drain the tank and measure that or drain it and fill to the level you are measuring at...
Even that would be beside the point. For our purposes, "empty" means "the point of fuel starvation," regardless of how much gasoline actually remains in the fuel system.

A completely dry tank might take on a gallon or two before the low-pressure pump makes reliable pressure.

To geek out properly, he should run the van until the engine quits. Don't overdue it and burn up the pump(s). Some codes may need resetting afterward. Bring a jerry can.
 

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Even that would be beside the point. For our purposes, "empty" means "the point of fuel starvation," regardless of how much gasoline actually remains in the fuel system.

A completely dry tank might take on a gallon or two before the low-pressure pump makes reliable pressure.

To geek out properly, he should run the van until the engine quits. Don't overdue it and burn up the pump(s). Some codes may need resetting afterward. Bring a jerry can.
Right, which is why I suggested running until the van stops to calibrate relative to the van in the post above. If you do that and can fill with some accuracy then you know your own tank "capacity" which means from topped off to van stopping. I think you and I are seeing this the same way.
 

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I have driven my Metris down to "40 miles remaining" with the needle pretty much at 0 about three or four times so far and each time it took between 18.5 and 19 gallons or so to fill back up, which means there was still about 2 gallons left in there as the total capacity is about 21 gallons. This is pretty close to usual for MB vehicles as the "reserve" fuel tends to be 2-3 gallons depending on the model and engine type.
 

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Discussion Starter #15
I have driven my Metris down to "40 miles remaining" with the needle pretty much at 0 about three or four times so far and each time it took between 18.5 and 19 gallons or so to fill back up, which means there was still about 2 gallons left in there as the total capacity is about 21 gallons. This is pretty close to usual for MB vehicles as the "reserve" fuel tends to be 2-3 gallons depending on the model and engine type.

So this may be the source of some of this confusion. The manual (p 307) says the Total capacity is 18.5 gal, but it then says "of which reserve fuel Approx 3.2 US gal". Now I would take the grammar of that sentence to suggest the 3.2 gal reserve is part of the 18.5 total, but if it were in addition to the 18.5 gal that would certainly make sense given some of our observations.
 

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Nope, it's part of. I've ran my van out of gas. Said I had 50 miles to go before I was on the side of the road. That was a year or 2 ago so I don't quite recall, but it was close to 18 gallons I added from the pump (minus what came out of the jug to get me there). The miles to empty is simply incorrect. Maybe they use the same float as a sprinter or C class and didn't calibrate it for our vans. Same reason our DST isn't correct on the radio. It's just not a vehicle they are worried about.
 

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Most I've ever put in the tank was 18.3 gal. I do not recall the estimated to empty miles.
...but it was close to 18 gallons I added from the pump (minus what came out of the jug to get me there)...
So the "Total Capacity is 18.5 gal": dead on.



..."of which reserve fuel Approx 3.2 US gal"...
And reserve of 3.2 gal is part of 18.5.

My conclusion: when the warning is on I would have 90 miles driving max at 55 MPH OR 50 miles at ~80 MPH.
 

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I've fueled multiple times and put around 18.5 gallons in and it was running fine.....:surprise:

Mine is actually at the warning light currently, I'll fuel it up tomorrow and see how much it takes vs its remaining range and will post here.
 

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Well I did my fuel up, just as I said, sitting just about at 0 on the gauge, running just fine....358 miles on the trip odo for this tank. Here's the pics:


If the tank was 18.5 gallons it would have been running pretty bad having had only 0.049 gallons left. MPG was 19.39, which is mostly city driving, less than 20% freeway.
 

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