So I keep hearing people referring to the "lie-o-meter" when discussing the dash MPG readout. For the life of me, I cannot understand this attitude.

In modern, fuel-injected vehicles, the computer tunes exactly how much fuel is injected in each cylinder for every revolution. It knows exactly how much fuel is being used. It also knows how many tire revolutions are being counted by the odometer. It's a pretty simple calculation for the computer to make.

Compare this to a human being squeezing in fuel at whatever pump is handy, filling till it clicks off once, maybe a few times, at different temperatures with different brands of different pumps at different gas stations. Even if it's the same exact pump at the same exact gas station every single time, it's not like the fuel sloshing around is going to be anywhere near as exact as the very specific amount of fuel injected by the computer.

Of course, if the speedometer is off (due most often to changing tire brand, size, or wear) then the odometer will be off, so whether by human hands or electronically the numbers will be off in accordance with the speedo.

So why is it exactly that so many people have convinced themselves that their very rough estimations resulting from the massively variable fuel station pumps are so much more accurate than the exact amount of fuel injected into the engine by the computer? Why have these same people perpetuated the myth that the computer is intentionally lying to you or otherwise has very little idea as to how much fuel it's injecting into the cylinders at any given time?

 

There is no adtiude twards the lie-o- meter. Do simple math and you'll see it's wrong. My 00 ex says I avg 15.5 city, hand calculated I avg 13.2 city. Highway it says I avg 16.4 after hand calculating it comes to 18.6 avg. ( 500 mi trip, 65 no cruise, flat)

 

My speedo was off from the factory, so the "truth(?)-o-meter" was being mislead.

Since I've upgraded to 35's, the speedo's right on, and my mpg's are reflecting correctly (hand calculated a couple times to verify).

 

Assuming everything is calibrated correctly, it's a simple matter of mathematics, calculus versus algebra.

The computer uses calculus to measure fuel use dynamically as the fuel is consumed. This shows mileage dropping as the vehicle accelerate or increasing as the vehicle coasts when the accelerator is not used.

Most people simply calculate total miles traveled/gallons of fuel used. This gives only a static averaged mean of the fuel used.

 

My trucks lie to me every day...all day. The only thing that is accurate is the temp and direction. In the case of my F-450, it says it gets about 16 mpg unloaded and 12 mpg loaded. Both figures are incorrect.

In the case of the 7.3 liter engine....the PCM really has no idea how much fuel is being used. It performs a 'volume of fuel desired' calculation based on PW and ICP and a few other variables. Turbo drive pressure can be an indicator of how much fuel is being used but that assumes a full combustion event as well as no boost leaks. When the injectors have a lot of miles on them, then the overhead meter variance can be greater than when the engine systems are in good health. When my injectors were old and worn out, my overhead meter was a compulsive liar. Now with new injectors, it just tells me little white lies.

 

Great point and I agree with you

I use the display in the dash as current and use the hand calc average as actual because the average captures the variability of driving conditions the best

 

My dash display appears to be a weighted average vs. an instant readout. When I sit at idle, it still shows 12-15 mpg, depending on the driving I had done for the last couple hundred miles.

And it's reported mpg differs from what I calculate based on miles driven and gas purchased. If the difference were a small fraction of an mpg, it could be attributed to various measurement uncertainties. But it is more like a half of a mile per gallon, which is significant % difference. I know how accurate the state folks are when they certify the pumps - they exhibit the equipment at various events.

Based up the fact that the display also shows "mile to empty," I had thought the vehicle computer was measuring gas in the tank vs gas delivered to the cylinders.

Does anyone know where this measurement is actually made? I'm asking about a gasser if that makes a difference.

 

most of the errors people see in the fuel mileage readout comes from the way it is reset, or not reset, in many cases.

the ONLY way to get the computer to correlate with hand calcs is to reset the meter immediatly before starting the engine. if it is reset at any other time, there will be a built in error. It should start off every tank at 0.0mpg. then at the end of the tank, fill the tank to the automatic shutoff and quit. anything else and it wont correlate.

and it probably still wont correlate because the pumps aren't perfect. they calibrate the pumps to a specific volume (usually 5 gallons), within an acceptable tolerance. double the volume, double the error.

add to that the volumetric relationship fuel has with temperature, and you have introduced another degree of variablity. sure, you got exactly 5 gallons, ... at 10 degrees ... that same volume of fuel at 70 degrees will occupy a different volume and will no longer be exactly 5 gallons. what temperature was the pump calibrated at? what was the temp when you filled your tank? you'd have to know these factors inorder to get a true measurment of fuel added to the tank.

the computer gets around most of this by using a known pressure, a more consistant temperature, and by measuring mass instead of volume. volume is back calculated from mass, and is going to be more accurate because the conditions by which it was measured is more constant and controled.... if it weren't, the engine wouldn't run correctly.

so ... bottom line.... barring some programming error in the back calculation algorithm, the lie-o-meter is more accurate. filling up at the pump is a pretty good guess, but there are too many uncontrolled variables to call it "better" or "correct".

if you really want to check your lie-o-meter's accuracy. here's how to do it:

1:start with a bone dry tank, and fill up at a pump which was just calibrated.
2:add only the amount of fuel to the tank at which the pump was calibrated (5gal)
3:make sure you are filling at the exact same temperature at which the pump was calibrated -OR- do the Volumetric correction for the temperature difference.
4: drive till empty and stop immediatly.

then check the numbers.

unless you are doing all of that, then you are really only guessing.

 

meborder's post has the right idea.

For some of the complaints by others, I think maybe there's a misunderstanding of what that readout is intending to indicate. It is giving you average fuel usage since the last time it was reset. You've traveled a certain distance, and a certain amount of fuel was used to get you over that distance. Therefore it will show you the overall MPG since the last time it was reset. It is NOT an instant readout unless you have that great info screen in the '11 and newer trucks, which I love, that gives you both instant as well as average (including averages over the last 5 resets) as well as trip averages, lifetime average, and so on.

The DTE calculation (Distance To Empty) in my '11 is not as dynamic as I'd like, but it is calculated at when you start the truck. It takes the approximate amount of fuel in the tank, looks at whatever the average MPG is at the moment, and gives you a number of how long it'll take you to run out at that mileage, meaning it has to assume that same average will continue to be accurate for the duration of your trip. In the '11 and newer you can set it to calculate based on towing or empty, which is cool. I wish it'd be willing to reevaluate its DTE calculation more often (even if it's just once every 5 minutes), but it doesn't know your intention, so it's just a best guess. Unlike passenger cars, you may have been pushing the rating limits on the way up to a destination and then be coming back empty, making a massive difference in fuel consumption, and it doesn't really know that, so it's just going on what info it does have: average MPG versus fuel in the tank.

Maybe the 7.3L PCM is more primitive than I thought, as I've never owned one myself, but I'd be surprised if it's just making very rough guesses about how much fuel it's injecting. It doesn't matter how thoroughly the fuel burns or whatever, only how much fuel is being sucked out of the tank and spit into the cylinders. The read-out on my 6.0L seemed to be pretty accurate, and I have absolutely no reason to doubt the readout on my current 6.7L...

 

How is the mass measured, with what accuracy?

 

How 'rough' or 'refined' the guesses are depends on a variety of conditions. For instance, if the ICP sensor fails the CCM test then the PCM ignores that value and substitutes one from a look up table. If one of those table values is boost, then drive pressure is important....drive pressure depends almost entirely on how much fuel is being burnt and does not account for any fuel that passes through (rolling coal, etc...). Without a known ICP value from the ICP sensor then the PCM may not calculate load correctly and may actually be asking for more IPR duty cycle to compensate. Then we have the variable of nozzle flow, which can be restricted over time. So, injectors with a couple hundred thousand miles may not flow as well as a new injector and the PCM has no real way of detecting this.

All this might be different if there was some sort of fuel flow sensor on the 7.3 but there is not. As far as the PCM is concerned, fuel is a static known value that never varies. In reality, fuel pressure, flow and delivery is quite dynamic when observed over the course of tens of thousands of miles. I have observed the data on my overhead console to be quite inaccurate during the period when I was driving around on old injectors. After the nozzles were changed out then the MPG data suddenly became much more realistic. In fact, it may be very accurate but I haven't yet burned through enough fuel tanks to check the figures for certain.

 

Injector Dynamics - GM Data

please forgive the clear infraction of brand loyalty, but the info is solid.

 

The DTE estimate is based off the indicated fuel level in the tank as read by the sending unit, and a multi-tank average fuel economy algorithm which tries to account for the way the truck is used most often. The DTE also tries to account for the type of fuel being burned as well. for example, switch from regular gasoline to E85 and you can see the DTE change dramatically within the first few seconds of travel. It is not based on the average fuel economy currently displayed, but rather a more "historical" look at the type of use the truck has been seeing.

i wouldn't give you two-cents for the DTE estimate if the truck sees a wide range of use, e.g. very heavy towing to highway driving and everything in between. the DTE will eventually account for the type of use the truck is currently seeing, but the data will ultimately converge at Zero ... which is a bad place to be, so i never really look at the DTE ... to gimmicky, IMO. I do tend to fill up any time it says "low fuel" regardless of how much fuel is in the tank, but that's just playing it safe on my part.

the DTE readout is actually more dynamic than you might be thinking.

for a fun experiment: with about a half tank, start the truck on the level and note the DTE. then sit idling with the nose down (10-15 degrees is plenty) for a couple minutes and watch the DTE number nose-dive.

then, turn the truck around and let it idle nose-up and watch the DTE rebound to a number significantly higher than the display showed while level. also take note of how long it takes to rebound. they have padded the readings so that it won't display erroneously high numbers when driving up a long grade. it will take about twice as long to read high as it will to read low.

how do i know this ... days at work can get long sometimes ...

 

Thank you!

It clearly explains that fuel is controlled by opening the injector for a desired amount of time, and then goes on to show how erroneous this can be if injector characteristics are different than assumed.

The mass of fuel is NOT measured at all - which would explain why the displayed MPG and hand calculated MPG rarely agree.

The hand calculated MPG is based on a measured amount of fuel (although a few folks have correctly pointed out many sources of measurement error.)

In the example in your referenced article, the initial calculated injector opening time delivered only 85% of the fuel expected - a huge error.

 

Depends on how you look at it. mass of the fuel is measured via injector pulse-width from a known injector flow rate. the mass of the fuel is measured the same way the mass of the air is measured, by flow rate. To say it is "not measured at all" is highly misleading. if it were not measured, the engine could never attain stoic conditions.

Very true, but this is a known condition and is accounted for by what they are referring to as "offset"
"This offset exists with all injectors and needs to be accounted for if want our fuel calculations to be accurate. Because this offset is constant across the majority of the injector operating range, we can correct for it by adding it to our calculated pulsewidth value."

as the article states, the problem is exacerbated below 2ms pulse width, however this too is known and accounted for by what they are calling the "low pulse adder table"

"With the addition of these values we can achieve nearly perfect fueling down to practically zero flow, and the PCM can do its job of calculating and commanding the correct air fuel ratio under all operating conditions.

It seems, to me at least, fair to say that the PCM knows very well how much fuel, (down to nearly 1/100,000th of a gram), has gone through the injectors.

I realize i'm not going to change anyone's opinion, but to insinuate that the gasoline dispenser, designed to handle and measure gallons, is more precice than a fuel injector designed to handle and measure thousandth's of a gram is just down right silly.

and to think that a simple miles/gallon equation is more accurate than the complex algorithms used to calculate injector pulse width is sillier still.

JMHO.