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Old 03-23-2014, 03:55 PM   #29
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Originally Posted by CampDaven View Post
The computer provides a somewhat educated guess, and does not account for fuel use by the generator or Aqua Hot. Semi-accurate MPG is done over an extended period using a calculator. ...
Actually the computer does exactly what it's supposed to do, which is calculate fuel mileage (mpg) as used by the engine. As stated above, it does not measure fuel used by the aqua-hot, or generator. But it is accurate regarding the fuel consumed by the engine.

Since it's measuring flow rate of fuel consumed by the engine, it's way more accurate than hand calculations, which obviously do take into consideration all other fuel used, such as by the generator, or aqua-hot system if so equipped.

Here's the catch: The pre-2006 Freightliner computers which came with the 'dim display' are not resettable. It averages fuel consumed by the engine from day one. Therefore it's averaging in early engine break-in mileage, etc.

My coach (2007) does have the resettable Freightliner computer, and I find it more accurate than any calculations I can do, based on all the reasons others have posted, such as fuel foaming at fill-up, and never getting to the same 'full' level every tank fill. I reset my 'tank' mpg calculation at every fuel fill-up.

I don't know about the other computers, but the newer FL computers are great. I actually have 3 different mpg calculators on my FL display.

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Old 03-23-2014, 04:04 PM   #30
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Originally Posted by Coma View Post
Cool feature! If you did that only on legs where there was no generator or AquaHot usage I sure you could get pretty close.
Yes, when you fill up the unit displays how much fuel it thinks you've used. When that matches the number on the pump (and magically, it does after a while) then... you're there. At that point the MPG readout is about as accurate as anything is going to be (exclusive of other fuel-using components of course.)

The device also displays any of several engine management parameters (via OBDII or other data bus) in real time as you drive. You can even program it to display proprietary custom data fields if the manufacturer makes the codes available.


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Old 03-23-2014, 04:25 PM   #31
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My computer in my truck and car are pretty accurate because they are gas powered the tanks are small and fill the same every time.

This isn't true, for at least a diesel pusher. My previous coach was a Monaco with a 7' wide tank that was mounted from side to side. If the coach was not level during fill ups, it could be off by 5 gallons either direction.

I use the Silverleaf monitor to track mileage and use a pencil and paper to compare. I believe you can't get accurate mileage in a DP without at least 10-15 fill ups in a row, but can be close. I believe in tracking my mileage for all the reasons listed. By running the Silverleaf "Recent Mileage gauge" I can track how the coach is doing instantly. It will often tell me we're climbing a subtle grade that can't be seen or there is a headwind. I often adjust my driving habits as such, saving me fuel.

The other thing that is forgotten about diesel fuel tracking is that often a diesel pumps fuel to the engine and quite a bit of it is returned to the tank. To me, the best way to track diesel mileage would be run it to and from the engine via two flow meters. Subtract the difference and you would know exactly how much fuel was being used.

I had a flow meter in my boat that was accurate to 3/10's of a gallon in a 40 gallon fill up. It was a great safety issue in a boat.
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Old 03-23-2014, 07:55 PM   #32
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I take my MPG (instant or avg) witha huge helping of salt. I noticed the miles-remaining-to-empty varies greatly. For one point, it depends on whether the rig is on the same plane for every fill and whether you fill it the same way every fill. Both are nearly impossible. But my miles-to-empty may read 450 immediately after a fillup. 30 miles down the road it may read 435 or 460, then 100 miles further, it may read 400. Again this may also be a function of the plane of the tank at any measurement moment. I say plane of the tank rather than attitude of the tank, because I believe my tank has a bad attitude.

Whatever the variables, the rig gets around 5.5 - 6 mpg. Usually, or sometimes...
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Old 03-23-2014, 08:52 PM   #33
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Our MPG by the computer is 7.1. Our scan gauge read 8.2. Neither takes into account what the generator uses.
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Old 03-25-2014, 12:13 PM   #34
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Diesel fuel foams a lot and you will need to check several tanks of fueling to be able to get a somewhat accurate reading of fuel consumption. It is important since a drop in 1-2 MPG average can mean problems w/ engine, exhaust manifold, turbo charger,etc that may need attention. I have had those problems w/ my Gasser and Diesel Motorhomes.
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Old 03-25-2014, 01:05 PM   #35
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Originally Posted by Snomas1 View Post
Diesel fuel foams a lot and you will need to check several tanks of fueling to be able to get a somewhat accurate reading of fuel consumption.
Yes, it can be challenging (or at least time-consuming) to truly top-off diesel tanks due to the foaming. Practically, you just can't do it at every fillup, making keeping a log of MPG difficult. That's one reason to invest in a Scanguage or other accurate aftermarket MPG gauge. Once calibrated (and if you keep them calibrated) they are pretty darn accurate, maybe better than you can do be hand, at least in the real world. Especially since a manual method can't take into account generator usage, but with an accurate MPG gauge you can shut down the generator, reset the gauge, and drive for a while on level ground. The average MPG figure from the gauge will give you a reasonably good figure to work with (Again, this is with a good, calibrated aftermarket gauge. The stock ones usually aren't that great.)

To get them calibrated you need consistency in determining the amount of fuel added to the tank. Actually seeing the fuel level in the tank filler is the bast way, but as mentioned that may not always be possible on large diesel tanks. Another way that works pretty well is to fill at less than the maximum rate and use when the nozzle first shuts off. That will be fairly consistent and while that may not fill your tank it will give you a pretty good tank-to-tank refill quantity, and that's all you need for calibration. Once done you need only repeat the process once in a while to keep things calibrated and not worry about it on every fillup, yet another reason a good MPG gauge is sometimes better than trying to do things by hand.
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Old 03-25-2014, 01:42 PM   #36
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Reading this thread seems like only the DP owners are interested in their MPG. I am in the same opinion of one of the earliest posts and that being "who cares now". Of course prior to last season I tracked my MPG as accurate as I manually could. I wrote down in a log at every fill up, whether it was 6 gallons to top off the tank, so I could work off a full tank calculation after a trip, or fill from near empty. Now, I just do a quick calculate for reasons stated to keep tabs, but no wore log etc. Why bother? My gas rig avgs. between 7.5-8 or 6.5-7 with or with out tow +/- either way. I find myself less concerned with full use now, just pull out the Discover Card and fill'er up!
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Old 03-25-2014, 02:51 PM   #37
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One reason I keep track of mileage my 'current trip' mileage is to determine how many miles I'm going to get out of a tank of fuel. Seven MPG vs eight MPG is a difference of 100 miles if you have a 100 gallon tank. A big head or tail wind can make that difference.
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Old 03-25-2014, 02:54 PM   #38
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Mine is dead on.

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