"Distance to Empty" indicators - why don't they align with actual miles driven?

[kphoger]

On my Pontiac, once it gets low enough that the fuel light comes on, the miles-to-empty display won't show anything but "LOW FUEL", probably to keep people from relying on it for precision it doesn't have.

This bothers me.  Just at the point that I need to be most concerned about how many miles I can go before filling up, it stops giving me usable data.

Given the vagaries in the accuracy of these indicators, it could be the case that the data was never all that usable. Precision versus accuracy and so on.

I also get the automakers' concern about Johnny Hotrod who if he had that more specific information would deliberately drive past a couple gas stations thinking he can still squeeze out a few more miles and end up running out earlier than anticipated. And we all know those people do exist.

What actually happens in my car:  DTE counts down to about 30, then just displays "LOW" from that point on.

What should happen (and did happen in my previous car):  DTE counts down to zero, then just displays "LOW" from that point on.

If the car manufacturer is worried about Johnny Hotrod running out of gas, then simply make the DTE count down to zero with a couple of gallons still remaining.  Oh, wait, they already do that anyway.  You can still drive a car even after the DTE gets down to zero;  in fact, your car's manual might even tell you that, and ma-a-a-aybe even how many gallons are left at DTE=zero.

Unless I'm on a road trip with pre-planned gas stops, I almost never fill up with gas till the low fuel indicator comes on.  Probably half the time, I let the DTE get down to "LOW" before filling up.  If it gets to "LOW" halfway home from work, I know I've still got at least enough fuel to run an errand first before filling up.  And I've never once run out of gas, ever in my life.

Finally, on a related note, why does the indicator always seem to display a different "Distance to Empty" figure every time I get gas? Any insight is welcome!

How consistent are you about filling to shutoff?  A necessary but not sufficient precondition for a miles-to-empty value that does not vary much from one fillup to the next is filling the tank to the same level every time and promptly resetting the trip computer.

I have never been able to figure out whether the Camry uses a preconfigured value for fuel economy to estimate miles to empty before actual consumption data becomes available following a reset, or if it uses a value calculated from previously collected data.  Either is technically possible, though the former is much simpler to program, leads to more predictable results, and places no demands on memory storage.

As for me, I almost always fill up at the same gas station, usually even the same pump, and I always fill up till the pump shuts off—never topping off.  So my baseline is pretty consistent.  And I've noticed that the DTE number changes from fill-up to fill-up.  And yes, it shows fewer miles to empty if I got worse fuel economy with the previous tank.

This is actually spelled out in my owner's manual:  "The fuel range estimate is based on an average of the vehicle's fuel economy over recent driving history and the amount of fuel remaining in the fuel tank. This estimate will change if driving conditions change. For example, if driving in traffic and making frequent stops, this display may read one number, but if the vehicle is driven on a freeway, the number may change even though the same amount of fuel is in the fuel tank. This is because different driving conditions produce different fuel economies."

[DriverDave]

My car does the same thing. The starting number after getting gas is usually accurate to my cars fuel range. But it will bounce around all over depending on whether I am doing local or highway driving. Sometimes it changes up to 160 miles in the same day, even if I only drove 20-30 miles. The better of my 2 cars gas mileage, my Honda Civic, gets about 38-40 mpg on the highway. But the indicator doesn't bounce around as much.