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Car Forum / Lexus Cars / November 2006RX350
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 > 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.  SignatureRay O (correct punctuation to reply) > 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. >> 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 [quoted text clipped - 5 lines] > 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. SignatureRay O (correct punctuation to reply) > 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 > 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? 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 > 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 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 > > 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 > [quoted text clipped - 12 lines] > > 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? >> > I made this same observation two years ago about my 2004 ES330. > See: [quoted text clipped - 29 lines] > 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. > 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. SignatureRay O (correct punctuation to reply) > 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? >> The gauge will not move down in a linear fashion because the float for > the [quoted text clipped - 9 lines] > > 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. > 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. SignatureRay O (correct punctuation to reply) > 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?" > 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? >> I think that the people who are confused by the current setup >> would be even more confused if the gauge display were [quoted text clipped - 4 lines] > > Seems to me that linearity would be better than nonlinearity. Yes, I agree. > 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. >> The gauge is somewhat calibrated to account for the movement >> of the float on the sender. The top and bottom quarters on the [quoted text clipped - 3 lines] > 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. SignatureRay O (correct punctuation to reply) > 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. >> To understand why the fuel gauge on the instrument panel >> moves the way it does, it helps to understand how the fuel [quoted text clipped - 8 lines] > (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. > 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 [quoted text clipped - 3 lines] > 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. SignatureRay O (correct punctuation to reply) > 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. > 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. >> 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. [quoted text clipped - 5 lines] > 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. >> 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. SignatureRay O (correct punctuation to reply) > The error factor in the "distance remaining" display varies greatly > depending on how the vehicle is being driven. There are so many factors [quoted text clipped - 3 lines] > 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. > 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 error factor in the "distance remaining" display varies greatly >> depending on how the vehicle is being driven. There are so many factors [quoted text clipped - 7 lines] > 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 >> You probably have about 2 gallons remaining when the low fuel warning >> light starts to come on. [quoted text clipped - 4 lines] > > 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. SignatureRay O (correct punctuation to reply) > >> The error factor in the "distance remaining" display varies greatly > >> depending on how the vehicle is being driven. There are so many factors [quoted text clipped - 15 lines] > 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. > >> You probably have about 2 gallons remaining when the low fuel warning > >> light starts to come on. [quoted text clipped - 9 lines] > starting the car. I am pretty sure this is pointed out in the owner's > manual. > 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. > 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. Ignore anything except the gas gauge. You can do it! > 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! > 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! 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 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.... |
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