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

[KCRoadFan]

Yesterday morning, I set out on a trip from Kansas City (where I live) to Albuquerque, and I drove 188 miles at the start before my first fill-up of gas, in McPherson, Kansas. I noticed that the "Distance to Empty" indicator on my dashboard, which was 362 miles when I left KC, read 236 miles when I went to get gas in McPherson - a difference of only 126 miles. Of course, my first thought was, "How did my car manage to gain an additional 62 miles of fuel capacity as I drove?"

Based on that experience, I figured I might ask: have any of you experienced a similar phenomenon with your car? (For the record, I drive a 2016 Toyota Camry.) Also, why doesn't the "Distance to Empty" indicator always correspond one-to-one with the number of miles you've actually driven, regarding the rate at which the number displayed decreases, and because it clearly doesn't, how does the car even calculate that figure? 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!

[freebrickproductions]

I suspect it has to do some with fuel economy, like how much you were coasting/braking, accelerating, running at a steady speed, climbing hills, etc. I've noticed mine does the same thing, and varies depending on if I've recently done a lot of non-stop/long-distance highway driving prior to filling-up.

[algorerhythms]

The number is a guess based on your fuel efficiency. If you do driving that is more efficient than your average driving (e.g, highway driving on a long downhill or level stretch at a moderate speed without any stop and go), it will be an underestimate. If you do driving that is less efficient (e.g. city driving), then it will be an overestimate.

[Rothman]

Yep, noticed the same thing with every car I've driven with the feature.  I just attribute the differnences to the fact that how much accelerating and coasting one does is unpredictable.

I also wonder how stable the burning of fuel is in one's engine (e.g., how hot the flame is, how the quality of fuel comes into play, etc., etc.).

[1995hoo]

It's inherent that those things can only be an estimate. Among other things, suppose you get stuck in a massive traffic jam and you leave the car running while stuck (say because it's very cold or very hot and you want the AC). Even if you had been averaging over 30 mph so far, your fuel economy will plummet when that happens (when you stand at idle, you're averaging zero mpg), thereby reducing the range. As others have noted, the converse is also true; mine often climbs considerably after I get on the highway. That makes sense to me, given that I'll typically get 250 to 300 miles in all-city driving and 400 to 450 in all-highway driving.

I've long suspected that those things also leave some buffer as to the actual distance to zero so that someone who pushes it won't run out of gas right as it hits zero. I believe I read somewhere that someone figured out that in most Hondas and Acuras, when it hits zero miles to empty you actually have about two gallons of gas left.

[Scott5114]

Yep, the calculations are just capacity of tank ÷ measured gas mileage × percentage of tank full. If your car's has the ability to reset the gas mileage to zero, you can watch the distance to empty bounce all around as you accelerate and coast. It'll stabilize the more data it has in it.

I've long suspected that those things also leave some buffer as to the actual distance to zero so that someone who pushes it won't run out of gas right as it hits zero. I believe I read somewhere that someone figured out that in most Hondas and Acuras, when it hits zero miles to empty you actually have about two gallons of gas left.

Probably so. 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.

[webny99]

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!

I'm not sure I understand what you're asking here, but to take it very literally... there isn't going to be a gas station within the exact same mile range every single time you need to get gas. 

I've long suspected that those things also leave some buffer as to the actual distance to zero so that someone who pushes it won't run out of gas right as it hits zero. I believe I read somewhere that someone figured out that in most Hondas and Acuras, when it hits zero miles to empty you actually have about two gallons of gas left.

Yes, there is definitely some buffer there, and I'm sure it varies by vehicle, but two gallons seems like a lot. That's almost 15% of the capacity of a standard 15-gallon tank. 0.2 gallons, correlating with 3-5 miles depending on the vehicle and trip type, seems more realistic.

EDIT: some quick research suggests that perhaps most vehicles have more buffer fuel than I thought. My main takeaway is that it seems to depend on the reader type. Anything that displays a number value of miles to zero is probably closer to accurate than a fuel "light" (which obviously doesn't mean 0 when it turns on or it'd be useless) or an old-style gauge (which will often fall well below E before truly running out).

[1995hoo]

All I know for sure is that my Acura has a 17.1-gallon tank, and while I've run it low enough on more than one occasion for "Miles to Empty" to show zero, I've never come close to putting in 17.1 gallons. I'm pretty certain I've never even put as much as 16 gallons in the tank at any single fillup. My wife hates it when I drive with the yellow light on indicating low fuel. She'd probably go nuts if she knew I go past the zero indicator on the miles to empty.

(The closest I've ever been to empty in a car was years ago when I borrowed my brother's 1974 Beetle and he warned me to fill the tank ASAP, so I stopped at a gas station a mile from home. It had a 10-gallon tank and it took 9.6 gallons. But, of course, that car didn't have a miles-to-empty gauge.)

[tmoore952]

This may or may not be related -- I'm honestly not sure --- but I have noticed that when my tank says 1/2 full, I have usually driven more than 1/2 of the miles that it takes to get to the "need gas" yellow light coming on (and when I say "more" I mean it, it's sometimes 60%+).

EDIT - I have a 2012 Subaru Outback with a manual fuel gauge, and so I am talking "manually" here.

My initial thought is that the tank might not be uniformly shaped, in the sense that the top of the tank might be wider than the bottom, such that if the surface of the gasoline pool drops (say) 1 inch from when the tank is full to when the tank is half full, it might drop (say) 2 inches from when it is half full to when it is empty (result being that more gas is burned from when the tank is full to when it is half full).  If the gas gauge is simply a measure of how far the surface of the gasoline pool has fallen, and the tank is shaped as I have described, that could explain the observation in my first paragraph.

Something similar (one way or another) might be going on with the "miles to go" indicator -- maybe that indirect estimate of tank fullness plays into it.
(However, in my car I usually look at MPG, not miles to go, so I have no direct observations of "miles to go" behavior).

[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.

[J N Winkler]

Yes, there is definitely some buffer there, and I'm sure it varies by vehicle, but two gallons seems like a lot. That's almost 15% of the capacity of a standard 15-gallon tank. 0.2 gallons, correlating with 3-5 miles depending on the vehicle and trip type, seems more realistic.

From the standpoint of fuel pump longevity, you want a large buffer (two gallons is actually pretty typical) since the pump depends on the fuel itself for cooling and lubrication.  Fuel pumps are very difficult to replace (depending on model, you have to go through the back seat or drop the tank) and have traditionally been designed to last for the lifetime of the car.

Based on that experience, I figured I might ask: have any of you experienced a similar phenomenon with your car? (For the record, I drive a 2016 Toyota Camry.)

I drive a 2005 Toyota Camry on roadtrips.  It behaves the same way:  miles to empty decreases monotonically as I drive, but--on the highway--typically at a slower rate than miles actually driven.

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.

[Amaury]

I think the distance to empty after topping off on fuel depends on what kind of driving you've been doing and your average miles per gallon. Our 2020 Toyota Corolla SE that we got in August 2019 has ranged from around 380 to around 490 miles per gallon. When we first got it, it was staying below 400. There was a while where after topping off, we were getting between 450 and 490 miles per gallon, usually somewhere in the middle. Lately, it's been staying around 430 miles per gallon after topping off.

For our 2022 Toyota Tacoma TRD Off-Road that we got in March 2022, I haven't seen anything higher than around 350 miles per gallon after topping off yet. It has ranged between 325 and 350 miles per gallon after topping off, and right now it's usually been around 335 miles per gallon after topping off.

[hotdogPi]

That's by far the most fuel efficient car I've ever heard of.

[1995hoo]

....

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.

I'm pretty certain that my Acura uses some sort of calculation that takes recent fuel economy into account. When I fill up the tank in routine driving (which these days happens maybe every six weeks to two months), typically the initial "miles to empty" figure will be no more than 300 miles. When I fill up the tank on a longer trip after doing all-highway driving, the figure is more likely to be over 400 miles. I think I've seen it go as high as 468 miles, though the furthest I've ever gone on a single tank in that car is 455.2 miles. Could have gone farther except I was in an area I didn't know very well and I didn't want to take chances.

My wife's much newer Acura has thrown a figure of over 600 miles to empty several times, although the farthest we've ever made it on a single tank was 534 miles.

[Flint1979]

I've gotten my Ford Fusion up to 601 miles before on the range. It's a hybrid. Lately I've had it saying about 550 miles when I have it topped off. I kind of ruined my gas mileage on a trip up to the U.P. last week when I was doing 85 miles an hour on I-75.

[TheHighwayMan3561]

I very rarely push the limits of my car's fuel, but earlier this year when I did the RTE shut itself off around about 30 miles of estimated range.

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.

I get that, but 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.

[CtrlAltDel]

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.

[webny99]

For our 2022 Toyota Tacoma TRD Off-Road that we got in March 2022, I haven't seen anything higher than around 350 miles per gallon after topping off yet. It has ranged between 325 and 350 miles per gallon after topping off, and right now it's usually been around 335 miles per gallon after topping off.

That's by far the most fuel efficient car I've ever heard of.

Probably per tank, not per gallon.

[sprjus4]

I think the distance to empty after topping off on fuel depends on what kind of driving you've been doing and your average miles per gallon. Our 2020 Toyota Corolla SE that we got in August 2019 has ranged from around 380 to around 490 miles per gallon. When we first got it, it was staying below 400. There was a while where after topping off, we were getting between 450 and 490 miles per gallon, usually somewhere in the middle. Lately, it's been staying around 430 miles per gallon after topping off.

For our 2022 Toyota Tacoma TRD Off-Road that we got in March 2022, I haven't seen anything higher than around 350 miles per gallon after topping off yet. It has ranged between 325 and 350 miles per gallon after topping off, and right now it's usually been around 335 miles per gallon after topping off.

490 miles per gallon???? I need this car, right now!

[Henry]

I find this feature handy in my 2019 Equinox. If I can get it up to a shade over 400, then I call it a successful topping off. Plus, I hate driving in notorious rush-hour traffic, and my ability to work from home helps out a lot too, so I'm free to use it more for road trips.

[ErmineNotyours]

A couple of years ago I failed to get enough gas on my way up to Johnson Ridge Observatory at Mt. St. Helens.  I wanted to go by Seaquest State Park on the way home.  Johnson Ridge has Wi-Fi, and Google said the nearest gas station going that way was all the way on I-5 at Castle Rock.  So I turned on economy mode, and turned on the gas range feature.  Of course, the mileage varied widely depending on if I was going up or down hill, but luckily it was mostly down.  Then at Toutle, 11 miles before Castle Rock, I found a gas station that Google didn't know about, and I was saved.  I probably would have had enough gas anyway.

[Takumi]

The low fuel light on my Acura turns on at about 50 miles to empty. When I owned a Lexus GS300, the low fuel light for that came on at 1/8 of a tank.

[KCRoadFan]

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!

I'm not sure I understand what you're asking here, but to take it very literally... there isn't going to be a gas station within the exact same mile range every single time you need to get gas. 

That's not what I meant. What I meant was: whenever I fill up, why does it show 370 miles one time when I start the car after filling up, or 418 miles another time, or some other random number on some other occasion? How does the car determine that?

[Scott5114]

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!

I'm not sure I understand what you're asking here, but to take it very literally... there isn't going to be a gas station within the exact same mile range every single time you need to get gas. 

That's not what I meant. What I meant was: whenever I fill up, why does it show 370 miles one time when I start the car after filling up, or 418 miles another time, or some other random number on some other occasion? How does the car determine that?

"Because your MPG data is different each time" remains the answer.

[GaryV]

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!

I'm not sure I understand what you're asking here, but to take it very literally... there isn't going to be a gas station within the exact same mile range every single time you need to get gas. 

That's not what I meant. What I meant was: whenever I fill up, why does it show 370 miles one time when I start the car after filling up, or 418 miles another time, or some other random number on some other occasion? How does the car determine that?

"Because your MPG data is different each time" remains the answer.

Plus the other previously given answer - how much did that pump top off before stopping?

"Miles to empty" has to be based on previous driving history. Whether that's the last 10 miles or last 100 miles or last 1000 miles is up to the algorithm the programmers at the manufacturer devised. The car can't predict how you are going to drive next. It only knows what happened before your fill-up.