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Computer Range Calculation - Empty at 70

10K views 25 replies 13 participants last post by  focus805 
#1 ·
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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#3 ·
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.
 
#12 ·
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.
 
#13 ·
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.
 
#17 ·
...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.
 
#5 ·
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...
 
#14 ·
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.
 
#15 ·
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.
 
#16 ·
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.
 
#18 ·
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.
 
#19 ·
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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#21 ·
I usually do....but the van said 45 miles remaining (so about 2.5 gallons) right before it switches to the picture of a van next to a gas pump.

I still say the tank is likely a "total capacity" of around 20.5-21 gallons.

On my W126 420SEL the published capacity is 21 gallons, but when I ran it REAL low once I fit in almost 23 and it was still going....21 + 3 reserve = 24 total

This same amount discrepancy has held true on essentially every MB I've ever owned. (which is 8 at this point)
 
#22 ·
In my experience the difference is the octane of the fuel filled with. The range is more true when filled with 93 versus 87. The avg mpg is 2-4.5 difference depending on how spirited your driving habits are. But it is clear to me that the Metris (as well as most MB engines) are designed to run on 93 exclusively. Do what you will with my anecdotal observations.
 
#23 ·
I have that range variance and I run 93 almost exclusively, rarely filling with 91 when I can’t get to my local cheap station (Conoco) and what I find has it. In most cases I will just put enough in to get to my local Conoco because it is usually the cheapest base price to begin with and my ConocoPhilips card and app gets me an extra 25 cents off the cash price.
 
#24 ·
The fuel mileage computer and low fuel reserve warning light are neither accurate nor precise. I don't care about the fuel mileage computer as there is a reliable alternative in simple arithmetic. The low fuel reserve warning light is too unreliable to gamble on 3.2 remaining U.S. gallons. I paid $7.99/gallon recently in the desert near the AZ-CA border as I was 30 downhill miles into the yellow light and in too much of a panic to wait for AZ gas prices. The tank took 16.24 gallons. So in this instance the reserve capacity is at least plausible -- no way I'd trust it though.
 
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#25 ·
Hah. I've paid the price for filling up near the CA/AZ border -- on I40. Going east bound I usually fill up in Barstow then drive east and fill up in Kingman AZ. This is 206 miles. Even in my Hellcat not a problem.

And the flip side is heading west I fill up in Flagstaff then approx. 160 miles later I fill up in Kingman and drive to Barstow.

The few times I have for some reason or another "had" to fill up in Needles or even at a station just inside AZ I paid the price. There ain't any gasoline more expensive than Needles gasoline.
 
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