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1st half of the tank of gas lasts so much longer than the 2nd half

jeeperguy21

jeeperguy21

NAXJA Forum User
Location
Syracuse, UT
I know this is not uncommon with many makes of vehicles, but anyone know why it is this way? I drive around 100 miles before the gauge says I've even burned 1/8 of a tank, but then the rest of the 7/8 of a tank only lasts me for another 200 miles for an average of 300 miles per tank.

This really isn't a problem, but more of a curiosity type question. Just for random info, my Jeep is a '99. I suppose I could google this, but that would be no fun.
 
Typically, the fuel tanks are tapered slightly toward the bottom (tapered inwards.)

This means that the "lower half" of the tank is actually lesser in volume than the "upper half" in many cases! No, I don't know why this is done, but I've seen it on a number of vehicles.
 
I can't think of any vehicle I've ever had that the fuel guage didn't read as you described yours does. Some worse than others. As mentioned I think it may be a combination of tank shape and general inaccuratcy of the sending unit. I get a laugh of of guys who brag on their gas mileage:
Man I drove 150 miles on less than a 1/4 tank!! :doh:
Let's see em dirive 150 miles on that last 1/4 tank.:roflmao: ....Al
 
Last edited:
People who wonder why fuel guages are not linear don't read "Full" when Full get no serious answer from me today, actually most days.

Fuel guages are guides and are not meant to reflect reality. Infact I believe most fuel guages use a reference point outside this universe.
 
A few possibilities...

The arm on the float is a lever, with the sender out on the arm and up a little bit. At different angles the sender has different amounts of contact so the output signal is less precise when the arm is all the way up.

The upper limit on the guage is fixed (mine is a little past Full but others are closer to F). If you fill the tank past the gauge peg you'll just be pegging the gauge until it gets down a bit.

If you fill all the way to the top of the tube then you've added a couple of gallons to your top 1/4 tank. Heck I can drive around for a couple of day and stay on F the whole time.

The bottom of the tank has a smaller walled reservoir, so that gas doesn't move out of reach of the pump tube everytime you go up a hill. The reservoir area is only about half the size of the rest of the tank, so there is less gas to consume in that last 1/8th of the tank.
 
It sounds like the perfect guage. Who cares if it's inaccurate at the top of the tank. I want to know when I'm close to running out and having the higher resolution at that end is great. Enjoy it.
 
What I noticed when i bought my 2000 XJ is this. When the reserve light comes on, the gauge drops to empty over the next 10 minutes regardless of how far I drive. I don't care how much gas is in the tank when it's full. What I want is to have any clue how much is left for the last 1/8 tank. After the gauge drops, I can go at least 50 miles before I chicken out.
 
ehall said:
I got 400 miles on my last tank, @ 19.5 mpg

:D
Click to expand...

:(

I was sitting here feeling jealous, but then it dawned on me I got 700 miles out of my last tank. :eyes:
 
I have an answer from my Auto tech days for this question:

From Kirk Vangelder, Clark County Vocational Skills Center, circa 1997:

We as lazy americans like to have a little "reserve" capacity in the tank, plus we like to see better mileage. SO, OE manufacturers have figured out that if the sending unit is wound differently form top to bottom (of the guage reading) that we will be happier.

The first half tank goes by slowly to fool us into thinking that the vehicle is getting good mileage.

The middle of the guage reads at a normal rate, and the bottom reads at a faster rate, that way we think we are running out of fuel and will stop at the pump sooner.

If you ever run yours down to the empty line, than fill it, you may discover that you have about 2-3 gallons left in the tank.

Unless of course your is dented, than YMMV.

Now, my real life exrperience with this:

I have in fact purposely run my XJ dry to test how big the tank is. I got another 50 miles out of the tank reading solidly on empty. AT roughly 12 MPG (Yeah I know my mileage sucks) thats almost four gallons or so.

I also discovered my 20 gallon tank will hold a hair under 21 gallons before the pump kicks off......

 
I think there's also alot more fuel slosh when its closer to empty.
 
Ecomike said:
:(

I was sitting here feeling jealous, but then it dawned on me I got 700 miles out of my last tank. :eyes:
Click to expand...
Some of you diesel guys sure do have bragging rights when it comes to MPG. A guy I work with has a VW TDI and boasts 45-50 MPG highway and never lets me forget it.
 
So about four gallons in reserve is normal for Cherokee's? I've been watching my mileage, comparing receipts and noticed how wildly inaccurate the gauge seems to be. An 1/8th of a tank might be 1 gallon or 3 on any given day. Wondered if I had a bad sender or something, but I guess it's working about as well as it's ever going to?
 
88 Wagonman said:
Some of you diesel guys sure do have bragging rights when it comes to MPG. A guy I work with has a VW TDI and boasts 45-50 MPG highway and never lets me forget it.
Click to expand...

We all need some way to stroke and massage our egos!:D

I may have trouble passing a tortous with my diesel, but at least I don't have to stop for gas every 5 minutes.:D

Comes in handy when the gas stations all close, the freeways get gridlocked with dead parked cars (out of gas) during a hurricane evacuation like we had here in Houston not too long ago.

I thnk my diesel gets about 250 miles on the first 1/8 of the gauge, LOL.
 
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