Fuel gage non-linearity

Toyota has been doing this for quite a while - my husband's Celica is exactly the same. The first bar, as well as the last bar, both last a lot longer than they should. The next few last a bit longer, and then it seems linear in the middle.

I'm not sure why they've decided that they want you to actually have 3/4 of a tank at -1 or -2 bars, and 1/4 of a tank at the opposite end, but it's definately by design. My husband thinks it keeps him from running out of gas, but since neither of us have ever done that in any car, I find it hard to believe that's actually why they'd do it.

 

My 02 Tahoe and 04 Avalanche did the same thing. Not just Toyota.

 

The Prius gas tank is lined with a vapor-limiting bladder. That might well lead to the non-linear problem you've noticed.

 

Looks like they could shape the tank in such a fashion to promote a more linear drain... even if it was a hassle to do it electronically.

 

So, I certainly don't believe that the car gets better mileage when the tank is more than half full. HOWEVER, wasn't there some discussion on this board a while back about how you should keep your tank more than 1/4 full? Something about the cooling of the in-tank fuel pump? Now, it seems a little dangerous to me that you'd be using gasoline to transfer heat, but I guess that if there's not significant oxygen in the tank then you wouldn't have to worry about it?

Just something I read; I may be off base.

 

I saw a similar post here about tanks last week. Someone posted a picture of a modern gas tank and it's not a cylinder anymore but a very oddly shaped container meant to squeeze in in a tight space. I suspect this makes the meter be none linear as well. I also think they have a very simple meter in the tank and don't get a good reading.... so to compensate I'm sure they make it look full longer then drop fast...

My maxima does the same not sure what the big deal is... All I use the meter for is for a quick look to see if I need to fill it up right now. I'm not surw I want engineers to spend a lot of time on making the meter linear. I would rather have them spend time on real FE issues or maybe improve the seats

 

Why not improve the digital meter?

Providing you start off with a full gas tank:
Why can't the car tell you have many gallons you have used and how much left?

If the car already knows:
1. when you fill the tank
2. Consumption of gallons used "it knows exact mpg?"

There is complaint about the guage not being linear.... why can't it just show you how much is left and how much you have used?

And for those instances when you don't fill all the way to the top, you could add an entry.

 

I hear your points. I would like to see a "miles to empty" meter guesstimated from the MPG value used. But friends with car that has this tells me that its woefully inaccurate in their cars (BMWs) but still it might be a cooler gauge then what we have and this car is full of gadgets.

I think it comes down to investment more then conspiracy... You don't choose a car based on it's fuel gauge so while it has to be there I as a car manufacturer wouldn't pay it any attention.

 

\
\Yea.. Its no good at all is its not accuarate within reason.

 

Lemme turn this around and ask which and what car has a linear meter? As far as I understand it, it has never been linear analog or digital anywhere.

My Civic with the analog guage lets me run up to 150 miles before I even see the white fuel arm come down for a car that runs ~380 miles to the tank and that hasn't been a big problem since 1996.

It's a small example, but maybe the question is better why does any car not have a linear gauge?

 

i think Toyota is listening... i noticed the CH does not have a bladder. im sure that will probably help to get more consistant fillups. as far as gas gauge accuracy. i dont really know that i have had a gas gauge that was any better. none i can think of were linear in any real sense of the word.

 

part of the problem with using a bladder and then trying to get the fuel gauge to read accuratly is on the Prius the gauge is mounted in a tube on the inside of the tank with a connection to the bladder at the bottom to fill the tube. Inside the tube is a float with a magnet on it and around the tube is a coil of wire, as the magnet rides higher the resistance in the coil changes and that is what the meter circuitry uses to send the signal to the dash unit. Now realise that as the car uses fuel that the bladder doesn't contract in a "top down" manner it just contracts, so the amount of fuel in the float tube isn't "top down" either so it's just showing what is in the tube. So the bladder contracts and the gauge stays nearly same till the level starts to drop more so than in a conventional car. Hope this makes some sense.

 

That makes sense to me..... they just need to change thier technology and measure through different means.....

But the simplest I can think of is to use thier existing technology and use a structure "plastic or metal" the is shaped perfectly to give a linear reading.

It shouldn't take a rocket scientist... I could build one myself in a lab.

its simple physics and high school student could do it as his qraduation project.

 

This doesn't seem to be Prius specific. I've owned cars for longer than I care to tell and have never had one with a linear fuel guage. All of our current cars tend to go slowly for the 1st half tank and then rapidly down for the second half. If you look at a tank float guage you'll see that this is how the resistance coils are wound. I'm not defending the practice, but it seem ingrained into the auto world mentality.

 

I agree... every car I've ever owned does that too.... I assumed it was because the gas in the filler neck and the first inch or so of the surface of the tank had to be used up before the float could even "start" to move.

 

I know it sounds easy but realise every time you go around a corner the bladder changes shape with the fuel moving the whole bladder around. What I've come to realise is it's the way it is and I ignore it. It's a great idea that reduces emissions and that's the reason we have it. Yes there is a perception that it's a defect, but try and design a gas tank that when you fill it up doesn't release the volume of 10-11 gallons of hydrocarbon laden air to the atmosphere. And have a gas gauge the reads to the exact ounce of fuel remaining. No small feat.

 

I don't know what all the fuss is about the Prius gas gauge other than it sucks. Everyone knows it is not a very good indication of fuel remaining and use other means to gauge how LOW to go.

Re a car with a linear fuel gauge, the best one I ever had was my '99 Pontiac Gran Prix. The tank held around 16 gallons and when it was 1/4 down, all you could add to the tank was 4 gallons. It was the same at 1/2 and 3/4 where the add was 8 and 12 gallons. The car also had a trip computer that kept track of average MPH AND gallons used. If the trip computer said 12.7 gallons were used, when I filled up, the total add would be within 1/10 of a gallon.

I suspect the car had some kind of instrument that measured the fuel as it came thought the line. I would think the Prius has some kind of way to measure the fuel flow also.

I just bought a new '06 Prius after driving my '04 around 27K over a 2 year period. The fuel gauge is my '04 didn't make much sense for a 11.9 gallon tank. I almost never put more than 8 gallons in the tank with the last bar in the blinking mode. Never had enough guts to run the tank dry to see how much further I could go from the "blinking" mode.

I've only filled the tank on the new car once so far. The gauge was down to 2 solid bars and I was able to add 8.8 gallons. On the '04, with 2 solid bars I might have been able to only add 6.5 gallons. Another thing about the '04. With the tank full, we were usually able to drive perhaps 150 or more before the tank gauge drop a bar. On the new one, at 60 miles, the first bar disappeared. So, it appears that there is a little bit of difference between the gauges on the '04 verses the '06.