RX350

[The Visitor]

What is it with the fuel guage and the distance remaining display. It
seems to declare a shorter distance remaining than there is in reality.
All this assuming the declared fuel capacity is correct. Is there a
reserve fudge factor put in?

John

[Ray O]

Post by The Visitor
What is it with the fuel guage and the distance remaining display. It
seems to declare a shorter distance remaining than there is in reality.
All this assuming the declared fuel capacity is correct. Is there a
reserve fudge factor put in?
John

I would think that it is safer to under-estimate remaining distance than
over-estimate and leave you stranded. Running the vehicle out of fuel will
eventually ruin the fuel pump and can damage the catalytic converter.

[The Visitor]

Post by Ray O
I would think that it is safer to under-estimate remaining distance than
over-estimate and leave you stranded. Running the vehicle out of fuel will
eventually ruin the fuel pump and can damage the catalytic converter.

Well yes that is what it is doing. if I didn't have that number there i
would drive it down to 1/8 of a tank in town. But going by that number,
I have to fill up at the 1/4 mark and it is a bit bothersome. I dislike
hidden or built in reserves because you start to ignore things over time.

[Ray O]

Post by The Visitor

Post by Ray O
I would think that it is safer to under-estimate remaining distance than
over-estimate and leave you stranded. Running the vehicle out of fuel
will eventually ruin the fuel pump and can damage the catalytic
converter.

Well yes that is what it is doing. if I didn't have that number there i
would drive it down to 1/8 of a tank in town. But going by that number, I
have to fill up at the 1/4 mark and it is a bit bothersome. I dislike
hidden or built in reserves because you start to ignore things over time.

If you do not mind risking damage to your fuel pump and catalytic converter
by running the fuel tank low or empty, it is possible to increase the
distance indicated in the distance-to-empty display.

[The Visitor]

Post by Ray O
If you do not mind risking damage to your fuel pump and catalytic converter
by running the fuel tank low or empty, it is possible to increase the
distance indicated in the distance-to-empty display.

It seems to underestimate by quite a bit. Is this adjusted at the
dealership with their plug in 'thingy'? Or can I do it? I have never ran
a tank dry yet. But having this thing scare me into filling it up at the
1/4 mark is a bit much. Bkut if I could bring myself to not look at it,
then I I would run it down to the 1/8 mark. Not a big deal around here.

I have not had one of these discance remaining displays before. Sure
nobody wants it to overestimate, but to so grossly underestimate only
causes it to be ignored.

John

[David Z]

Post by The Visitor
What is it with the fuel guage and the distance remaining display. It
seems to declare a shorter distance remaining than there is in

reality.

Post by The Visitor
All this assuming the declared fuel capacity is correct. Is there a
reserve fudge factor put in?

I made this same observation two years ago about my 2004 ES330. See:

http://groups.google.com/group/alt.autos.lexus/browse_frm/thread/ed89065
1cecb106a/d248d59cfc7e8d58?tvc=1&q=lexus+gas+gauge+david#d248d59cfc7e8d5
8

[The Visitor]

Post by David Z
http://groups.google.com/group/alt.autos.lexus/browse_frm/thread/ed89065
1cecb106a/d248d59cfc7e8d58?tvc=1&q=lexus+gas+gauge+david#d248d59cfc7e8d5
8

So nobody really knew.

With my last car, an Audi, 1/4 was actually 1/4 of the tank.

If I fill up at 1/4 mark now, it takes 75 % of the stated capacity. Fine.

But the distance to empty number says something like 75 km. (I'm i
Canada) but based on my mileage, I should be able to to 135 km to empty.

If it were not for the distance to empty, I would happily run it down to
1/8 of a tank in town. But that number, Grrrrr, scares me into untimely
fill-ups.

Or is the stated capacity of the tank wrong?

John

[David Z]

Post by The Visitor

Post by David Z
I made this same observation two years ago about my 2004 ES330.

See:
http://groups.google.com/group/alt.autos.lexus/browse_frm/thread/ed89065
1cecb106a/d248d59cfc7e8d58?tvc=1&q=lexus+gas+gauge+david#d248d59cfc7e8d5

Post by The Visitor

Post by David Z
8

So nobody really knew.
With my last car, an Audi, 1/4 was actually 1/4 of the tank.
If I fill up at 1/4 mark now, it takes 75 % of the stated capacity. Fine.
But the distance to empty number says something like 75 km. (I'm i
Canada) but based on my mileage, I should be able to to 135 km to empty.
If it were not for the distance to empty, I would happily run it down to
1/8 of a tank in town. But that number, Grrrrr, scares me into

untimely

Post by The Visitor
fill-ups.
Or is the stated capacity of the tank wrong?

One way to figure it out is to fill your tank and set your trip odometer
to zero. Then observe how many miles the range gauge counts down for
each 10 miles of driving. If the trip gauge counts down linearly for
the whole tank of gas, then it's probably not the cause of the
distortion. If it's not linear, then Lexus/Toyota has deliberately
skewed the range gauge so that people don't run the tank down too low.

Having just thought of it, I may try this idea myself sometime. The
problem is if your driving is not consistently highway, street or a
consistent mix of the two, your measurements will be distorted,
accordingly. Also, we don't know how the range gauge is programmed.
For example, does it assume a 50/50 ratio of highway/street driving or
what?

[Ray O]

Post by David Z

Post by The Visitor

Post by David Z
I made this same observation two years ago about my 2004 ES330.

http://groups.google.com/group/alt.autos.lexus/browse_frm/thread/ed89065
1cecb106a/d248d59cfc7e8d58?tvc=1&q=lexus+gas+gauge+david#d248d59cfc7e8d5

Post by The Visitor

Post by David Z
8

So nobody really knew.
With my last car, an Audi, 1/4 was actually 1/4 of the tank.
If I fill up at 1/4 mark now, it takes 75 % of the stated capacity.

Fine.

Post by The Visitor
But the distance to empty number says something like 75 km. (I'm i
Canada) but based on my mileage, I should be able to to 135 km to

empty.

Post by The Visitor
If it were not for the distance to empty, I would happily run it down

to

Post by The Visitor
1/8 of a tank in town. But that number, Grrrrr, scares me into

untimely

Post by The Visitor
fill-ups.
Or is the stated capacity of the tank wrong?

One way to figure it out is to fill your tank and set your trip odometer
to zero. Then observe how many miles the range gauge counts down for
each 10 miles of driving. If the trip gauge counts down linearly for
the whole tank of gas, then it's probably not the cause of the
distortion. If it's not linear, then Lexus/Toyota has deliberately
skewed the range gauge so that people don't run the tank down too low.

The gauge will not move down in a linear fashion because the float for the
gauge moves in an arc like the float in a toilet tank, and the tank has an
irregular shape. The primary goal of the fuel tank's design is to fit in a
particular space and to be crash-worthy. Shaping the tank so the gauge
would move in a linear fashion would reduce the tank's capacity and the
vehicle's range.

Post by David Z
Having just thought of it, I may try this idea myself sometime. The
problem is if your driving is not consistently highway, street or a
consistent mix of the two, your measurements will be distorted,
accordingly. Also, we don't know how the range gauge is programmed.
For example, does it assume a 50/50 ratio of highway/street driving or
what?

I believe that the MPG display is derived by calculating distance traveled
and dividing it by an estimate of how much fuel was consumed, either from
injector pulse duration or from a flow meter in the fuel system. With the
same amount of fuel in the tank, a driver with a light foot will get better
MPG than a driver with a heavy foot or who is driving in stop-and-go
traffic. The system then uses MPG to calculate distance to empty. You can
increase the distance-to-empty reading at any time by driving at a steadier
speed than you were just before.

[David Z]

Post by Ray O
The gauge will not move down in a linear fashion because the float for the
gauge moves in an arc like the float in a toilet tank, and the tank has an
irregular shape. The primary goal of the fuel tank's design is to fit in a
particular space and to be crash-worthy. Shaping the tank so the gauge
would move in a linear fashion would reduce the tank's capacity and the
vehicle's range.

Are you talking about the gas gauge or the range gauge here, or both?

Shouldn't both gauges be calibrated to account for the shape of the
tank?

[Ray O]

Post by Ray O

Post by Ray O
The gauge will not move down in a linear fashion because the float for

the

Post by Ray O
gauge moves in an arc like the float in a toilet tank, and the tank

has an

Post by Ray O
irregular shape. The primary goal of the fuel tank's design is to fit

in a

Post by Ray O
particular space and to be crash-worthy. Shaping the tank so the

gauge

Post by Ray O
would move in a linear fashion would reduce the tank's capacity and

the

Post by Ray O
vehicle's range.

Are you talking about the gas gauge or the range gauge here, or both?

In the paragraph above, I am talking about the fuel gauge.

Post by Ray O
Shouldn't both gauges be calibrated to account for the shape of the
tank?

I think that the people who are confused by the current setup would be even
more confused if the gauge display were calibrated to account for the shape
of the tank because the scale on the gauge would appear to be random.
The gauge is somewhat calibrated to account for the movement of the float on
the sender. The top and bottom quarters on the gauge are smaller than the
middle quarters.

[David Z]

Post by Ray O
I think that the people who are confused by the current setup
would be even more confused if the gauge display were
calibrated to account for the shape of the tank because the
scale on the gauge would appear to be random.

I don't know why you say that.

Seems to me that linearity would be better than nonlinearity.

How would linearity "appear to be random?"

Post by Ray O
The gauge is somewhat calibrated to account for the movement
of the float on the sender. The top and bottom quarters on the
gauge are smaller than the middle quarters.

Not sure what you mean here. Are you saying that means that when the
fuel gauge reads 1/4 of a tank, that there's actually less than 1/4 of a
tank of gas in your car?

[Ray O]

Post by David Z

Post by Ray O
I think that the people who are confused by the current setup
would be even more confused if the gauge display were
calibrated to account for the shape of the tank because the
scale on the gauge would appear to be random.

I don't know why you say that.
Seems to me that linearity would be better than nonlinearity.

Yes, I agree.

Post by David Z
How would linearity "appear to be random?"

I did not say that linearity would appear to be random. I said that the
scale on the gauge would appear to be random.

Post by David Z

Post by Ray O
The gauge is somewhat calibrated to account for the movement
of the float on the sender. The top and bottom quarters on the
gauge are smaller than the middle quarters.

Not sure what you mean here. Are you saying that means that when the
fuel gauge reads 1/4 of a tank, that there's actually less than 1/4 of a
tank of gas in your car?

No, the scale on the fuel gauge for the top 1/4 and bottom 1/4 is not the
same as the scale for the middle 2 quarters.
To understand why the fuel gauge on the instrument panel moves the way it
does, it helps to understand how the fuel sender moves. Think of the socket
where your upper arm meets your shoulder as the switch, your hand is the
float, and your arm is the connection between the float and the switch. If
you stick your arm straight out and raise and lower your hand, your hand's
movement is not straight up and down; you hand is moving in an arc, as
illustrated by Michelangelo's drawings. In other words, the float's
movement is not linear; it moves in an arc but the level of fuel goes up and
down horizontally.

[David Z]

Post by Ray O
To understand why the fuel gauge on the instrument panel
moves the way it does, it helps to understand how the fuel
sender moves. Think of the socket where your upper arm
meets your shoulder as the switch, your hand is the float.

Yes, I got the picture from your analogy to the float in the upper part
of a toilet.

But I think you're assuming that either:

(a) the fuel gauge must rotate in direct proportion to the "float"
device, and/or
(b) the only way to get the fuel gauge to be accurate is to distort the
geometry of the display.

What I'm saying the that there's a mathematical formula that will
convert the relative movement of the float to yield the correct volume
of fuel in the tank and that it would be relatively easy to program the
fuel gauge for that formula without changing the display. I don't know
whether or not the fuel gauge is designed that way, but it seems to me
that's the way it should be designed. Otherwise the fuel gauge is not a
very accurate instrument.

[Ray O]

Post by David Z

Post by Ray O
To understand why the fuel gauge on the instrument panel
moves the way it does, it helps to understand how the fuel
sender moves. Think of the socket where your upper arm
meets your shoulder as the switch, your hand is the float.

Yes, I got the picture from your analogy to the float in the upper part
of a toilet.
(a) the fuel gauge must rotate in direct proportion to the "float"
device, and/or
(b) the only way to get the fuel gauge to be accurate is to distort the
geometry of the display.

Yes to a and b.

Post by David Z
What I'm saying the that there's a mathematical formula that will
convert the relative movement of the float to yield the correct volume
of fuel in the tank and that it would be relatively easy to program the
fuel gauge for that formula without changing the display. I don't know
whether or not the fuel gauge is designed that way, but it seems to me
that's the way it should be designed. Otherwise the fuel gauge is not a
very accurate instrument.

The fuel sender-fuel gauge is a much simpler setup. Basically, the sender
is a rheostat and the gauge is the load in the circuit. A more accurate
system would add cost and complexity for little return for the automaker.

Rather than rely on the miles to empty readout, just watch the fuel gauge
and watch for the low fuel warning light.

[David Z]

Post by Ray O
The fuel sender-fuel gauge is a much simpler setup. Basically,
the sender is a rheostat and the gauge is the load in the circuit.
A more accurate system would add cost and complexity for
little return for the automaker.

OK. But I'm still curious to know what the error factor is. In other
words, when the fuel gas says 1/4, how much gas is really in the tank?
And why is there a 50% error factor between the fuel gauge and the range
gauge? That's no small difference.

Post by Ray O
Rather than rely on the miles to empty readout, just watch the
fuel gauge and watch for the low fuel warning light.

I agree. That's what I do.

[Ray O]

Post by David Z

Post by Ray O
The fuel sender-fuel gauge is a much simpler setup. Basically,
the sender is a rheostat and the gauge is the load in the circuit.
A more accurate system would add cost and complexity for
little return for the automaker.

OK. But I'm still curious to know what the error factor is. In other
words, when the fuel gas says 1/4, how much gas is really in the tank?
And why is there a 50% error factor between the fuel gauge and the range
gauge? That's no small difference.

The error factor in the "distance remaining" display varies greatly
depending on how the vehicle is being driven. There are so many factors
that affect the actual distance that the vehicle will travel on a given
amount of fuel that IMO, the "distance remaining" figure is pretty useless
information. About the only time that the distance remaining figure is
meaningful is when the vehicle is cruising at a steady speed with no gear
changes.

You can see the efffect of driving style on the distance remaining by
cruising at a steady speed, note the distance remaining, then disengage the
overdrive and you will see the distance remaining figure drop. Re-engage
the overdrive, and the distance remaining figure will actually increase.

Post by David Z

Post by Ray O
Rather than rely on the miles to empty readout, just watch the
fuel gauge and watch for the low fuel warning light.

I agree. That's what I do.

You probably have about 2 gallons remaining when the low fuel warning light
starts to come on. If you know what your average MPG is, you can guess how
far you can travel when the light comes on. For example, I get about 20 MPG
so when the light comes on, I figure 40 miles to empty.

[The Visitor]

Post by Ray O
The error factor in the "distance remaining" display varies greatly
depending on how the vehicle is being driven. There are so many factors
that affect the actual distance that the vehicle will travel on a given
amount of fuel that IMO, the "distance remaining" figure is pretty useless
information. About the only time that the distance remaining figure is
meaningful is when the vehicle is cruising at a steady speed with no gear
changes.

I expect it to use the average fuel economy figure in the calculation.
It consitantly tells me I will be empty 40 odd kms too soon. Just if it
was within 10 at least but 40+ is too much. But yes, I am stuck with it.

Post by Ray O
You probably have about 2 gallons remaining when the low fuel warning light
starts to come on.

I didn't know I had one! In my Audi it came on with 12 litres remaining
(within 1/2 litre) as was stated in the book. I would fill it up right
away, (lots of close stations) and it really was 12 l remaining.

John

[Ray O]

Post by Ray O
The error factor in the "distance remaining" display varies greatly
depending on how the vehicle is being driven. There are so many factors
that affect the actual distance that the vehicle will travel on a given
amount of fuel that IMO, the "distance remaining" figure is pretty
useless information. About the only time that the distance remaining
figure is meaningful is when the vehicle is cruising at a steady speed
with no gear changes.

I expect it to use the average fuel economy figure in the calculation. It
consitantly tells me I will be empty 40 odd kms too soon. Just if it was
within 10 at least but 40+ is too much. But yes, I am stuck with it.

I am pretty sure that it is using instantaneous fuel economy and not average
fuel economy to calculate distance to empty. Knowing how Toyota engineers
think, their logic would be that how the vehicle is being driven now is a
more valid indication of what the remaining distance is than how the vehicle
was driven before because it ispossible that a change in drivers or driving
style will have a bigger effect on remaining distance than past driving
style

Post by Ray O
You probably have about 2 gallons remaining when the low fuel warning
light starts to come on.

I didn't know I had one! In my Audi it came on with 12 litres remaining
(within 1/2 litre) as was stated in the book. I would fill it up right
away, (lots of close stations) and it really was 12 l remaining.
John

The low fuel warning light only comes on when the fuel is low and does not
come on with the other trouble lights when you turn the ignition on without
starting the car. I am pretty sure this is pointed out in the owner's
manual.

[Jay Somerset >]

On Tue, 14 Nov 2006 16:29:52 -0600, "Ray O"

Post by Ray O

Post by Ray O
The error factor in the "distance remaining" display varies greatly
depending on how the vehicle is being driven. There are so many factors
that affect the actual distance that the vehicle will travel on a given
amount of fuel that IMO, the "distance remaining" figure is pretty
useless information. About the only time that the distance remaining
figure is meaningful is when the vehicle is cruising at a steady speed
with no gear changes.

I expect it to use the average fuel economy figure in the calculation. It
consitantly tells me I will be empty 40 odd kms too soon. Just if it was
within 10 at least but 40+ is too much. But yes, I am stuck with it.

I am pretty sure that it is using instantaneous fuel economy and not average
fuel economy to calculate distance to empty. Knowing how Toyota engineers
think, their logic would be that how the vehicle is being driven now is a
more valid indication of what the remaining distance is than how the vehicle
was driven before because it ispossible that a change in drivers or driving
style will have a bigger effect on remaining distance than past driving
style

Not the instantaneous value, but a short-term average over the past few
minutes. Averaging over the entire trip/tank is meaningless if conditions
are changing. Audi's system seems to be a bit more accurate than
Toyota/Lexus, but if your driving conditions change, waiting 5 minutes
should give an adequately accurate estimate of "distance remaining" in all
cases.

Post by Ray O

Post by Ray O
You probably have about 2 gallons remaining when the low fuel warning
light starts to come on.

I didn't know I had one! In my Audi it came on with 12 litres remaining
(within 1/2 litre) as was stated in the book. I would fill it up right
away, (lots of close stations) and it really was 12 l remaining.
John

The low fuel warning light only comes on when the fuel is low and does not
come on with the other trouble lights when you turn the ignition on without
starting the car. I am pretty sure this is pointed out in the owner's
manual.

[David Z]

Not the instantaneous value...

Right. The instantaneous MPG fluctuates wildly (e.g., when you step on
the gas). The remaining miles is a very steady number.

...but a short-term average over the past few minutes.

I suspect that it uses the average MPG that's stored in the memory. And
if you reset the system to zero, it uses the average MPG since reset.

[David Z]

Post by The Visitor
I expect it to use the average fuel economy figure in
the calculation. It consitantly tells me I will be empty
40 odd kms too soon. Just if it was within 10 at least
but 40+ is too much. But yes, I am stuck with it.

My recollection is that it says 300 miles after a tank fill (it used to
say 320 miles, so I guess I'm driving more aggresively lately or driving
less highway miles). When the fuel gauge says 1/4 tank, I think it says
about 60 miles (75 to 80 miles would be 1/4 of 300 or 320). When the
low fuel light comes on I think it says 20 miles.

Looks like they're subtracting about 20 miles (as opposed to 20
kilometers). Maybe they just subtract 20 "whatevers" depending what
mode it's set on.

[mcbrue]

Ignore anything except the gas gauge. You can do it!

[David Z]

Post by mcbrue
Ignore anything except the gas gauge. You can do it!

Since you never have anything worthwhile to add to the discussion, just
ignore it!

You can do it!

[The Visitor]

Post by David Z
One way to figure it out is to fill your tank and set your trip odometer
to zero. Then observe how many miles the range gauge counts down for
each 10 miles of driving. If the trip gauge counts down linearly for
the whole tank of gas, then it's probably not the cause of the
distortion. If it's not linear, then Lexus/Toyota has deliberately
skewed the range gauge so that people don't run the tank down too low.

I'll look more closely. But at the beginning, at fillup. the dte is too
low. It should tell me about 584 kms. But last time said 503 km to
empty. When the reserve gets waaay to big, it creates problems of the
who thing being ignored. But they will come and bring me gas for free!

[The Visitor]

Fuel report. The gueage was reading 1/2. And it took 1/2 to fill, volume
measuered at the pump. 1/4 mark will be next.

John

[The Visitor]

Just to report I ran it down to below 1/8th mark. Well down.
It took 65L to fill up when the low fuel light came on.

This meant there was 7.5L in the tank that could get me 60 Kms. (city)
DTE reading was 37. So the error isn't so bad when I run it waaay down.

I will try it again for repeatability.

Just getting to know my car....