Why is the MPG computer not reliable?

[89sandman]

Originally Posted by JFXG

You check the mileage for the same reason that you check the oil pressure, tire pressure, air pressure, coolant level, batteries, lights, etc., etc. You're responsible for a large, complex machine that has your family in it, and the details can give you an indication of current health or impending problems.

this may have been true back in the old days but now with computer monitored systems I would say not so much anymore. the ecm will notice a problem long before you will ever notice any kind of gas mileage problem and alert you through the mil (malfunction indicator light). technology makes some things obsolete, such as this.

[tedgard01]

Frankly, it don't matter that much to me.

What is the big idea of "accurate MPG" anyway. Are you really going to trade in your coach if it is not getting the same MPG as someone else? It appears to be a big barging point to some folks to throw out a big MPG number...

The avg MPG is a fully accurate means of judging your avg performance. The manual calculations and mathematical computations that some guys go through ends up adding up to nothing of real substance or value.

If you want to impact your avg MPG in the most significant way, go 10 to 15 MPH slower, use a conscious effort to gain speed slower rather than use a lead foot approach, slow down sooner when approaching a stop, coast down hill when ever possible. Lose weight (take that any way that suits you). Any weight reduction will help MPG.

People get all wrapped up in the details, and forget that this is supposed to be a relaxing and fun way to spend your money.

[JFXG]

Originally Posted by 89sandman

this may have been true back in the old days but now with computer monitored systems I would say not so much anymore. the ecm will notice a problem long before you will ever notice any kind of gas mileage problem and alert you through the mil (malfunction indicator light). technology makes some things obsolete, such as this.

Technology has made it obsolete to pay attention and take responsibility for your family. Enjoy your travels, Eddie.

[SteveSkinner]

My Alladin is dead accurate.
Each time I fill I reset fuel remaining.
If I have to prepay my fuel I can tell pretty much to the gallon tell how much to add.
This confirms that the mpg will also be accurate.
It must also include generator fuel. My 12kw genset gets plenty of use.

[ARTWERKE]

Any method of checking mpg is subject to many variables, odometer accuracy,evaporation,accuracy of fueling station pump,generator usage(which is really an alternator, if accuracy is your mantra) so your scan gauge is as good as anything else.

[laj]

I never worry about fuel mileage at all. when it says close to empty I just fill it. life's to short to mess up the trip with petty things like that. screws up the whole experience, might as well stay home.

[hone eagle]

Mine is right on ,so close I have stopped double checking .They measure fuel injection 'pulses' to who knows how many decimal points including idle,the ECM also know temp baro pressure (altitude) and more we don't understand ,my transmission even relays load and hill angle (tilt)
9.5 mpg (avg) GCW 32,000#

[ohhwell]

These modern ones are pretty darn close! The computer knows just about how much fuel is going in and it knows pretty accurately how fast you are traveling. variations in tire diameter can affect it.

The calculator in our Prius is usually within 1mpg of my calculations. I would trust it more than my calculations based on how hard it is to get a consistent fill on that "tank". The MPG in our older Dodge 3500 was pretty right on as well (Unfortunately). HD vehicles and motorhomes should have a feel-good setting that adds a couple MPG to the readout. Many are going to inflate their numbers anyway.....

[smiller]

Originally Posted by 89sandman

the ecm will notice a problem long before you will ever notice any kind of gas mileage problem and alert you through the mil (malfunction indicator light).

That may be the case, but certainly not always. There are many mechanical malfunctions that could affect mileage and/or signal a looming problem that the ECU may or may not pick up.

[joeplazek]

The computers are right. They only know what fuel is used over the mileage traveled. Any other variable is up to you,,, Aqua Hot, Generator, idling, etc, . AND like said in other posts,,, the only reason to watch MPG is to reveal other troubles that happen along the way. You can tell a lot from a big rise in fuel consumption and get to the problem quickly. This could save you an unscheduled stop along some interstate in North America.

joe

[docj]

Originally Posted by joeplazek

the only reason to watch MPG is to reveal other troubles that happen along the way. You can tell a lot from a big rise in fuel consumption and get to the problem quickly

X2; I zero my onboard computer (Silverleaf) every day when we start and look at the MPG number occasionally as we drive and then at the end of the day. Mine ranges from 7.5-9.0 depending on a whole host of factors including terrain, wind speed, average MPH, etc. As long as it's in that range I know things are basically Ok. If it were to go below that for several days I would be concerned.

[Coma]

I don't know of any vehicle that actually measures its fuel usage. That is, has fuel flow measuring. The ECM calculates, based on the fuel map, how much fuel is supposed to be used, then does the math for MPG. Thats why they are not exact. If you could measure the difference between fuel taken out of the tank vs fuel returned then you could actually measure fuel usage. Then the calculation would be very close to actual.

[smiller]

Originally Posted by Coma

I don't know of any vehicle that actually measures its fuel usage. That is, has fuel flow measuring. The ECM calculates, based on the fuel map, how much fuel is supposed to be used, then does the math for MPG. Thats why they are not exact. If you could measure the difference between fuel taken out of the tank vs fuel returned then you could actually measure fuel usage. Then the calculation would be very close to actual.

That's what the ScanGauge units do, more or less. Their calculation is derived from miles driven vs. injector activity like all the others, but at every fillup you can enter the actual amount of fuel consumed. This trims the calculation to where it becomes very accurate.

[Coma]

Originally Posted by smiller

That's what the ScanGauge units do, more or less. Their calculation is derived from miles driven vs. injector activity like all the others, but at every fillup you can enter the actual amount of fuel consumed. This trims the calculation to where it becomes very accurate.

Cool feature! If you did that only on legs where there was no generator or AquaHot usage I sure you could get pretty close.