Fuel Tank Capacity - Low Fuel Light

[MarineHawk]

I base the estimate on the total life MPG of the vehicle. I have never reset Trip B, which means it shows the lifetime fuel economy and odometer of the vehicle. I understand what you are saying about how your mpgs can vary and thus your fuel economy estimate can change. My point is that I have had several instances where my mpgs didn't change and my remaining miles didn't change once the low fuel light came on; at least the estimate didn't change for several miles (i.e. saying I had 45mi left for about 20mi, then dropping down by like 5mi, 20mi, etc).

Regarding the LOW status, that is another bone I have to pick. If 10mi are left, then at BEST it can have about 1/2 gal left in the tank. I have run it a few miles AFTER it reached the LOW indication and still only refilled 19-20gal, which means as always, I am 2gal away from filling tank capacity, which should not happen.

I'm fairly certain that the computer bases it off of recent activity. For example, I can be driving locally in rush hour and stop-and-go (getting dismal gas mileage), and then fill up and head out on the highway. After I spend the first 50 miles or so, getting 20 MPG, instead of the previous 12 MPG, the Range actually is higher than when I started, even though I've burned through 2.5 gallons of gas. When, I'm driving down this one 7-mile stretch in WV with a 7% grade, getting "99 MPG," the Range ALWAYS goes UP from when I started the descent to when I ended it. I'm pretty certain that the range is based on recent activity, and not the lifetime mileage of the vehicle. Otherwise, it would NEVER go up. But mine goes up when I switch from low-MPG conditions to high-MPG conditions. I think it's based on a certain amount of prior miles. If I had to bet, I would say it's about 50 miles based on my observations.

I base the estimate on the total life MPG of the vehicle. ... My point is that I have had several instances where my mpgs didn't change and my remaining miles didn't change once the low fuel light came on; at least the estimate didn't change for several miles (i.e. saying I had 45mi left for about 20mi, then dropping down by like 5mi, 20mi, etc).

That would be consistent with what I'm saying, if, for example, you started hitting some downhill/uphill terrain.

I always reset Trip A when I fill up my tank. when I say the range is "going up," sometime that literally happens, but I also mean that to be when the miles spent on Trip A + the Range # goes up. If when I fill up the tank, it says the Range is 350 miles. If I have gone (according to Trip A) 100 miles and the Range says I have 300 miles left, that means the Range has gone up by 50 miles for that tank.

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[Rubicon Bob]

I know that I am not the brightest bulb in the box, but I think people are forgeting that the fuel level that is being reported to the computer is from the float attached to the fuel pump assembly. If the float is not adjusted properly, you will get false readings. Luckily, it is in our favor. It could be that you run out of gas with it showing a 1/4 tank left. Personally, I would rather have the extra gas in the tank, in case I ever needed it. Kind of an insurance policy. When I filled up today, the fuel gage was just below 1/4 tank. I got 15 gallons in before it shut off.

 

[Seansmd]

Two observations:
1) the low fuel must estimate off of a short term average not a long term average as some have highlighted. Failure to do this overstates range and probability of running dry.
2) fill amount depends on several things, the temperature, the tank angle, the pump sensitive to fumes to shut off the pump. Someone posted that after shut off, they waited and pumped in more, waited some more and pumped in more.

The truly scientific way would be run it dry then fill it from a pump with no shut off safety.

 

[PyrPatriot]

Two observations:
1) the low fuel must estimate off of a short term average not a long term average as some have highlighted. Failure to do this overstates range and probability of running dry.
2) fill amount depends on several things, the temperature, the tank angle, the pump sensitive to fumes to shut off the pump. Someone posted that after shut off, they waited and pumped in more, waited some more and pumped in more.

The truly scientific way would be run it dry then fill it from a pump with no shut off safety.

Any damage to the fuel pump if you run the tank to empty? I would suspect plenty of folks have unintentionally done so and thus manufacturers take the probability of it happening to one of their vehicles into account, and leave a bit of fuel in the pump/line as a reserve to prevent damage, if that is a thing

 

[Seansmd]

Repeatedly running a pump dry will damage it, can't imagine any designer not planning for it to happen on some level.

The other item i forgot to add to the observations is that Jeeps in particular expect to be in odd angles pitch and roll, so i would expect even more conservatism in the empty estimation.

 

[ShadowsPapa]

Repeatedly running a pump dry will damage it, can't imagine any designer not planning for it to happen on some level.

The other item i forgot to add to the observations is that Jeeps in particular expect to be in odd angles pitch and roll, so i would expect even more conservatism in the empty estimation.

My SX4 in-tank JEEP fuel pump has to deal with corners, stops, starts and hills when I run it below 1/4 tank - since it's got no baffles or system like the Jeeps have that keeps fuel there. If I run it low and take a fast corner the engine will sort of hiccup, then take off again. Never does that when 1/4 or more tank. So running it empty once won't hurt. Leaving ignition on when it's out of fuel won't be nice, but when it's out, shut it down. No problem.

 

[Mr._Bill]

The VERY first fillup my JT had (after test drive) the range estimator had 440mi at 19mpg. Which means there was 23gal in the tank. So there CAN be some variation. What concerns me is when the low fuel light comes on, because that is supposed to be at a VOLUME of fuel remaining. So if you fill it up when it comes on you should be getting 20gal of fuel in easy, not 18gal.

This is based on the assumption that you can actually get 22 gallons of gas into a 22 gallon tank. I don't know if that is possible, and does the stated tank capacity include the fill pipe? What about the effect of the vents, filters, safety systems, and the automatic shutoff at the pump?

The range estimate is based off a determined period of current driving, and not the lifetime average. I get high MPG and Range numbers when traveling, especially if there is a downhill slope. I check the numbers periodically, but don't monitor them while traveling. I know that I can go about 350 miles on a full tank, and try to plan my fuel stops to happen when the tank drops below half but before it goes below a quarter full.

I write down the MPG and mileage when I gas up, and then hand calculate the MPG based on distance and fuel use. The actual MPG usually comes in lower than the computer estimate. I also reset one trip meter at fill-up, and leave one running with the lifetime total.

 

[ShadowsPapa]

This is based on the assumption that you can actually get 22 gallons of gas into a 22 gallon tank. I don't know if that is possible, and does the stated tank capacity include the fill pipe? What about the effect of the vents, filters, safety systems, and the automatic shutoff at the pump?

The range estimate is based off a determined period of current driving, and not the lifetime average. I get high MPG and Range numbers when traveling, especially if there is a downhill slope. I check the numbers periodically, but don't monitor them while traveling. I know that I can go about 350 miles on a full tank, and try to plan my fuel stops to happen when the tank drops below half but before it goes below a quarter full.

I write down the MPG and mileage when I gas up, and then hand calculate the MPG based on distance and fuel use. The actual MPG usually comes in lower than the computer estimate. I also reset one trip meter at fill-up, and leave one running with the lifetime total.

Unless the JT is drastically different - here is what I've found -
Silverado with 24 gallon tank - when running on fumes and you have only 20 miles approx range left you can get 24 gallons in without filling clear to the cap.
Grand Cherokee - when the light comes on and the bell chimes saying low fuel and you track miles from when it came on and estimate you have under a gallon left, you CAN get full capacity in - I've done it twice, much to my wife's dismay (she HATES it when I let anything get that low, especially when she's with me or she is driving) I can't recall the book capacity of the GC tank, but two different Grand Cherokees, this 2018 and the one prior, I believe was a 2015 or 16, I've actually gotten full capacity in.
I can say the same for the Ford truck I had - dual tanks, and those would hold what the book said.
My SX4, and Javelins - same thing. I've had the fuel tanks off for restoration and other word so they were bone dry - and I could get the gallons specified into them.

So, unless the JT is different, that's tank capacity, not including the filler which is a distinct part from the tank.
Maybe they are talking Canadian gallons or Italian gallons or Imperial gallons or Nigerian gallons or something, I dunno - or, their sending units are not accurate. But that would be weird, too, as other Jeeps we've had and have, as well as my other pickups - you learned to trust those gauges and warnings. They were almost always spot-on.

I can normally go 400+ on a tank of gas. (unless pulling my EMPTY trailer but that's another thing)
On my SX4 I have used a spreadsheet as going by a single tank just never works. Angle of the vehicle, ambient temp, fuel temp, the exact spot you stop filling, and so on. I've found it varies between tanks more because of the inaccuracy of how the vehicle is sitting and other factors, so I have a spreadsheet that averages over multiple tanks.
I've not done that with the JT - but I've also found that once the nozzle shuts off I can actually sometimes get almost 2 more gallons in. This one is really weird that way. My others, once it shuts off you are pretty much done, maybe a half gallon more. But I've gotten quite a bit more in with the JT.

 

[PyrPatriot]

My honda element only had a 15.9gal tank but when I ran it low (like an estimated 1/4gal left based on 10 years of driving it) I fit 16.5gal into the thing before the autostop on the pump kicked in

 

[PyrPatriot]

Another attempt.
I was driving and noticed I was low on fuel. I saw the trip estimator read 15mi but the "Low" didnt show up like it is supposed to in its place. My preferred gas station was 15mi away and on the way as part of a longer trip. I make it just in time, and my avg mpg doesnt change. Fill up the tank, on slow flow and letting it go until auto-shutoff twice after 1min intervals. I got 20.5gal.

Upon filling up, trip estimator said 343mi and stayed there for about 30mi until it eventually started going down

 

[ShadowsPapa]

Another attempt.
I was driving and noticed I was low on fuel. I saw the trip estimator read 15mi but the "Low" didnt show up like it is supposed to in its place. My preferred gas station was 15mi away and on the way as part of a longer trip. I make it just in time, and my avg mpg doesnt change. Fill up the tank, on slow flow and letting it go until auto-shutoff twice after 1min intervals. I got 20.5gal.

Upon filling up, trip estimator said 343mi and stayed there for about 30mi until it eventually started going down

Mine usually shows "Low Fuel" indicator at about 28-30. Then after a while it goes back to the estimated miles left. OF course the color on the fuel gauge changes so you know it's low - the low indicator doesn't stay on forever - it switches back. Oddly, my Chevy once that was triggered it stayed on "low fuel" until I filled it again, never having any clue how much was left. I pulled into my driveway with my Chevy one evening - it said 55 miles left - no sweat, there's a station 10 miles away, another 3.5 miles if I go away from town. Next morning I got in and started it and it said low fuel! And the fuel gauge read almost on the E where the night before it was about 1/8 tank or a bit more.
At least this JT never does that BS to me. The chevy did that to me more than once- pull in the drive with 45 miles or more left, next day it says I'm empty.

I actually got 21.5 gallons in the JT tank once - normally I'm lucky to squeeze 20 in. I've gotten 20.5 a couple of times, but 21.5? It still said something like 15 miles left at that point.

 

[Grillguy40]

Today, I happened to accidentally run my tank down to E. Long story on how it happened. I was actually worried that it would start sputtering I was stuck in traffic trying to get to an exit for fuel. When I finally made it to a station and filled it, the most it filled to was 19.081 gallons. I was shocked given that I thought I had remembered that we have a 22 gallon tank. Guess I didn’t need to worry as much as I was.

 

[Mr._Bill]

The JT seems to underreport the amount of fuel in the tank. Maybe they are afraid of fuel pumps dying prematurely and having to replace them under warranty.

 

[Jonny A]

The JT seems to underreport the amount of fuel in the tank. Maybe they are afraid of fuel pumps dying prematurely and having to replace them under warranty.

I personally have never been able to put more than 18.5 gallons in at fill up. Even when indicated low fuel and all. Thought I would run out of fuel, too only have 3.5 gallons still. Ain't complaining.

We had a late 90's Dodge Dynasty in the day. May wife habitually ran it dry. Went through 3 intank fuel pumps. One under a warranty. Any way the mechanic said that they burning up prematurely probably from running low on fuel. My wife learned not to do that anymore. What a pain waiting on a tow even with AAA. No fuel pumps burned out since in our vehicles, 24 years.

 

[Jonny A]

I personally have never been able to put more than 18.5 gallons in at fill up. Even when indicated low fuel and all. Thought I would run out of fuel, too only have 3.5 gallons still. Ain't complaining.

We had a late 90's Dodge Dynasty in the day. May wife habitually ran it dry. Went through 3 intank fuel pumps. One under a warranty. Any way the mechanic said that they burning up prematurely probably from running low on fuel. My wife learned not to do that anymore. What a pain waiting on a tow even with AAA. No fuel pumps burned out since in our vehicles, 24 years.

Knock on wood. I hope I didn't jinx myself!!!

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