Gas gauge

[herarety]

Which do you believe, the needle or the scale?

As in, does the needle conform to the scale? Or does it fall constantly? Because if you've noticed, the gas gauge's first and last quarter are shorter than the middle two. I was just wondering if the needle slowed in the middle (which seems unlikely) or if it falls at a steady rate.

 

[Sir Nuke]

well, since it falls so slowly....not so sure if its SPEED is linear or not.....

the fact that MOST floats that I have seen in tanks are hinged at ONE end...and thus, since they operate through an arc, the amount that they physically move at the top and bottom of their range is less with relation to the drop in tank level...thus the needle moves less, thus the distance between marks on the gage is shorter....which would lead me to believe that the gage actually moves SLOWER in those regions.

I have noticed that if my driving is fairly constant...that I get right at 100 miles from one mark to the next.

(stash)

 

[Don]

The gas gauge isn't linear because the sides of the tank aren't parallel all the way to the bottom.

The top quarter is actually the biggest - Heck, I can get 90 miles before the gauge falls to the "full" mark. Half way seems to be about 2/3rds - I. E. 2/3rds of the gas is used up in the first 1/2 tank and the last half has about 1/3rd.

What does it matter whether it's linear or not. Just because you can go 300 miles on the first 1/2 doesn't mean you should begin making plans to fill up at 600 miles . . .

Don

 

[Sir Nuke]

Which do you believe, the needle or the scale?

so to directly answer your question....I BELIEVE where the needle points to on the scale...so BOTH! LOL

(stash)

 

[blynzoo]

Don's got it. The tank is not a perfect rectangle (The fuel wouldnt flow to the bottom, duh). And, like all other non-digital cars, The loss is exponential compared to the gauge, e.g. the gauge moves linearly (F to E) and actual fuel loss is a function of that and the shape of the fuel tank. My engineer uncle tried to explain this to me. Who freakin' knows.
Basically, as don said, half on the gauge is a bit over 1/3 left in the tank. I've driven the car to empty, you've got about 80-90 miles until the fuel light comes on.
Just keep track of miles traveled, dedicate one of your trip odometers every time you fill up.

 

[Sir Nuke]

although I agree that the tank is not a perfect rectangle and the sides are sloped.....I have done a lot of close observation of the gage when we went on our cross courntry trip....and the needle moves between each mark right at or close to every 100 miles...

to be honest...it REALLY doesn't make that much difference...for the simple fact that when the little light comes on...you only have about two gallons left.....or about 8 litres left in the tank......and if you drive your car between 50 and 60 miles after that light comes on (depending on your gas milage)...you will either be walking or hitchhiking..lol.

 

[roswel]

guage

based on this thread i'm assuming our guages are non-digital. that being said most guages of this type are calibrated so that the last quarter tank is the only time the guage is really accurate. this is really the only time you need to be seriously concerned about your fuel level IMO.

 

[BigBlue]

You get the funniest topics sometimes in here!

For me, I drive the car until the needle points to about 1/4 tank. Then I start thinking about getting a fill. If it gets to the point that my gas light comes on, then I REALLY know it's time to find a gas station.

 

[bmorton]

A GM engineer friend explained to me once that fuel gauges are calibrated to move slowly off of 'Full' because people like to drive around with a full tank for a while -- they don't like seeing that needle move off F ten miles after leaving the gas station. In other words, the movement of the needle isn't linear by design and so it's up to you to 'learn' how the fuel gauge relates to reality in each car you drive.

 

[Don]

although I agree that the tank is not a perfect rectangle and the sides are sloped.....I have done a lot of close observation of the gage when we went on our cross courntry trip....and the needle moves between each mark right at or close to every 100 miles...

I did some close observations on mine (just returned from a 1600 mile round trip this evening) and mine appears to move in 90 mile "chunks."

When filled to the top, it takes 90 miles for the gauge to move down to the full mark. Another 90 (180 total) gets it to the 3/4 mark. Another 90 to half, and another to 1/4. If I run it to just below the "E" (about 30 miles after the light comes on) I can just make the 5th 90, or 450 total - Yes, I get 31 MPG, give or take, driving at 75 with the car pretty well loaded, and the A/C on.

Don

 

[Don]

A GM engineer friend explained to me once that fuel gauges are calibrated to move slowly off of 'Full' because people like to drive around with a full tank for a while

No "calibrating" to it - If you fill the tank just enough for the float to hit the top of the tank, the gauge is pegged above the full mark. If you keep adding gas, the float is still hitting the top of the tank, and after adding another 2 or 3 gallons, the tank is now full.

When you begin driving, you have to use enough gas (2 or 3 gallons) for the float to not hit the top of the tank so the gauge can move down, even a little bit.

My P5 (as I mentioned above) goes 90 miles before the gauge comes down even with the "Full" mark.

Don

 

[bmorton]

The mechanics don't matter, the point is that it is the way it is because people like it that way, not because it has to be that way. It's fascinating how many things about a car are designed the way they are not because it's the best or most efficient way to do it but because that's the way people like it or what they're used to.

 

[Don]

I guess I can't explain ANYTHING worth a flip.

It's that way because it's the cheapest/easiest way to manufacture it - No other reason.

If you wanted a gas gauge which worked all the way to the top, you'd have to put a bump in the top of the tank for the float so it could keep rising as the gas completely filled the tank. Then, after you'd used even a small amount, it would beging falling. A better gauge, no doubt, but the costs associated with making it that way are not worth it to any manufacturer - They've got us so used to the cheaper way they do it, we even seem to like it

Don

 

[bmorton]

Don't blame yourself -- I don't think we're as far apart as you think. You're talking about non-computerised fuel gauges like the balancing coil type and I'm thinking of the newer microprocessor-controlled ones. Rather than depending on a calibrated friction brake on the tank unit, these can be programmed to compare the reading from the float position against a calibration curve to correct for the shape of the tank, provide damping for the needle, OR for the fuel gauge behaviour people expect (i.e., it stays at Full for quite a while).