How accurate is your lie-o-meter (vehicle based mpg meter)

[DT400]

Just curious how accurate everyone's dash mpg gauge is compared to hand calculations.
Mine is rather optimistic. It is 2.8% to 8.3% off and so far the total life of vehicle is 3% optimistic or about .5mpg off so far.
I know the meter can be calibrated but I'm going to wait longer before doing this, hopefully it will be a bit more consistent percentage wise as everything wears in and the computer has more data points to draw from.

Darrell

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[pkinneb]

Mine is about .5 to the optimistic side as well.

 

[Mtnman1]

Just curious how accurate everyone's dash mpg gauge is compared to hand calculations.
Mine is rather optimistic. It is 2.8% to 8.3% off and so far the total life of vehicle is 3% optimistic or about .5mpg off so far.
I know the meter can be calibrated but I'm going to wait longer before doing this, hopefully it will be a bit more consistent percentage wise as everything wears in and the computer has more data points to draw from.

Darrell

I think hand calcs are not 100% either. You have no way of knowing exactly how much gas is in the tank or how much you used.

The only way calculations would be 100% is to start with an empty tank. Run until empty.

 

[DT400]

I think hand calcs are not 100% either. You have no way of knowing exactly how much gas is in the tank or how much you used.

The only way calculations would be 100% is to start with an empty tank. Run until empty.

I disagree, you don't need to run till empty you just need to start from a full tank and refill to a full tank. using the same method and with the truck sitting in the same orientation. I'm not worried about usable gallons in the tank.
While your right hand calcs are not 100% accurate, after all your odometer may be off too but you can gps that for a test pretty easy. and sure you may be off .1 gal from fill-up to fill-up but here are things (as mentioned) you can do to mitigate that as much as possible.

Are you thinking the miles till empty (DTE) not miles per gallon??

Darrell

 

[Mtnman1]

I disagree, you don't need to run till empty you just need to start from a full tank and refill to a full tank. using the same method and with the truck sitting in the same orientation. I'm not worried about usable gallons in the tank.
While your right hand calcs are not 100% accurate, after all your odometer may be off too but you can gps that for a test pretty easy. and sure you may be off .1 gal from fill-up to fill-up but here are things (as mentioned) you can do to mitigate that as much as possible.

Are you thinking the miles till empty (DTE) not miles per gallon??

Darrell

When you fill your tank to full, you do not know how full. You will never hit the same point twice. It is an estimate. Did you put .5 gallons in more than when you started? No way of knowing.

The only way to be 100% accurate with hand calc is to start with an empty tank, put in 20, 23 gal. Whatever Drive till empty. That will give a constant to calculate. Miles driven divide by 23.

Filling to full with gas in the tank is a "fullish" estimate. You put in 22 gallons.
But you really have no idea how much was in there or how full you are. Where you at top tank? Half way up fill tube?

Will never be 100% accurate with how much gas was really used. Unless you have a clear tank with marks, or start empty and finish empty.

 

[Kanuck]

I wonder why someone would have to be that accurate on MPG or L/100 KM. It s an estimation at best. It is also an ongoing average of fuel consumption, which I have noticed in my truck it seems not to count idle time or fuel used.

 

[DT400]

When you fill your tank to full, you do not know how full. You will never hit the same point twice. It is an estimate. Did you put .5 gallons in more than when you started? No way of knowing.

The only way to be 100% accurate with hand calc is to start with an empty tank, put in 20, 23 gal. Whatever Drive till empty. That will give a constant to calculate. Miles driven divide by 23.

Filling to full with gas in the tank is a "fullish" estimate. You put in 22 gallons.
But you really have no idea how much was in there or how full you are. Where you at top tank? Half way up fill tube?

Will never be 100% accurate with how much gas was really used. Unless you have a clear tank with marks, or start empty and finish empty.

To be 100% accurate you would need a separate fuel tank filled with a properly measured amount of fuel done with calibrated equipment (probably done by weight and temp calculations) then driven over a very accurately measured distance then do the math.
Who is going to tow their truck to a gas station with a empty tank just to fill it up to the top???Then still as you say how do you know if it is the same amount of full?

Considering a pump shuts off when back-pressure hits the nozzle it will be pretty accurate if you do the two squeeze method waiting 10-15 seconds after the first shut off for foam to settle. Of course the truck needs to be in the same orientation (front/rear and side/side) for the first and measured fill-ups.
The filler neck is where the nozzle is and that's where the fuel would contact it to shut it off, The distance you put the nozzle in would be the same give or take 1/2", I am guessing the filler neck is no bigger than 2" ID. Since a 2" dia tube has 3.14 cubic inches per inch and a gallon is 231 cubic inches each inch of the filler tube represents approx 1/74th of a gal or just over 1.7 ounces out of 128 ounces/gal. I think our accuracy is close enough for this exercise.

Darrell

 

[Mtnman1]

To be 100% accurate you would need a separate fuel tank filled with a properly measured amount of fuel done with calibrated equipment (probably done by weight and temp calculations) then driven over a very accurately measured distance then do the math.
Who is going to tow their truck to a gas station with a empty tank just to fill it up to the top???Then still as you say how do you know if it is the same amount of full?

Considering a pump shuts off when back-pressure hits the nozzle it will be pretty accurate if you do the two squeeze method waiting 10-15 seconds after the first shut off for foam to settle. Of course the truck needs to be in the same orientation (front/rear and side/side) for the first and measured fill-ups.
The filler neck is where the nozzle is and that's where the fuel would contact it to shut it off, The distance you put the nozzle in would be the same give or take 1/2", I am guessing the filler neck is no bigger than 2" ID. Since a 2" dia tube has 3.14 cubic inches per inch and a gallon is 231 cubic inches each inch of the filler tube represents approx 1/74th of a gal or just over 1.7 ounces out of 128 ounces/gal. I think our accuracy is close enough for this exercise.

Darrell

My point is, hand calcs are "ish". So why do folks beleive the calcs over the computer? Truth is most likely somewhere between the 2.

I personally dont care enough to calculate.

Its a truck, whetever the real MPG value is, its gonna suck.

 

[Mtnman1]

To be 100% accurate you would need a separate fuel tank filled with a properly measured amount of fuel done with calibrated equipment (probably done by weight and temp calculations) then driven over a very accurately measured distance then do the math.
Who is going to tow their truck to a gas station with a empty tank just to fill it up to the top???Then still as you say how do you know if it is the same amount of full?

Considering a pump shuts off when back-pressure hits the nozzle it will be pretty accurate if you do the two squeeze method waiting 10-15 seconds after the first shut off for foam to settle. Of course the truck needs to be in the same orientation (front/rear and side/side) for the first and measured fill-ups.
The filler neck is where the nozzle is and that's where the fuel would contact it to shut it off, The distance you put the nozzle in would be the same give or take 1/2", I am guessing the filler neck is no bigger than 2" ID. Since a 2" dia tube has 3.14 cubic inches per inch and a gallon is 231 cubic inches each inch of the filler tube represents approx 1/74th of a gal or just over 1.7 ounces out of 128 ounces/gal. I think our accuracy is close enough for this exercise.

Darrell

And you still dont know how much is in the tank when you fill up, so dont really know how much is used.

 

[daemonic3]

For those that don't trust the computer and need absolute accuracy, you have to do it over many many tanks. Use an app like Fuelly (or track in your own sheet which seems like an insane amount of work) to enter in every time you get gas (ODO reading and gallons) and it will be the most accurate overall possible. The only pump error you introduce is in the most recent tank, and that error will get amortized across all your fillups until it becomes irrelevant.

 

[Bryan Simon]

Over the course of a year and not quite 15,000, my pickup has been fueled up at the same station, same orientation, and predominately the same pump.
Sometime within 3 months I did go into the engineering mode and adjust an extremely proud mpg reading.
At this point, the Lie-O-Meter and my hand calculations are very similar.
I fill til the first click off, and then add to the next even dollar.
(sometimes, not possible to add as it will over fill and spill out)
so, even with fuel @ $2.00, less than a half gallon difference in volume.

Currently averaging 19.2 with little city, mostly rural hiway, and some freeway.
Mileage takes a hit on the freeway—80mph not conducive to high mileage
(except in my 66 ElCamino with a wheezy 307 and 5spd and tall rear gearing—1900rpm, 25mpg @ 80mph)

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