Why aren't fuel gauges linear?

[JoeyP]

My car's gas gauge works like this, measured from 0 miles on trip odo:

Topped off tank: just above F, 0 miles
Needle on F: go about 40 miles from fill-up
Needle on half: go about 200 miles
Needle on E: go about 320 miles

So from F to half is about 160 miles, but from half to E is about 120 miles.

Why can't gauges be linear? So if I can get 300 miles off a tank, each quarter would be 75 miles. It doesn't matter too much, since I normally fill it around 1/4 remaining.

 

[Throckmorton]

To fool you into thinking your car is more efficient

 

[herm0016]

it is usually linear with the level in the tank, not the amount of fuel. the tank is not a rectangular prisim, it is an irregular shape.

 

[mmntech]

This explains it.
http://www.cracked.com/article_19571_8-gadgets-that-lie-to-you-every-day.html

 

[exdeath]

Like most things in engineering, we want higher resolution where it matters most even at the expense of something else.

 

[Zenmervolt]

This explains it.
http://www.cracked.com/article_19571_8-gadgets-that-lie-to-you-every-day.html

Actually, it doesn't. It's a valiant attempt and it seems like common sense, but it's not the actual reason.

The gauges in most cars are intentionally designed to stay at the "full" mark longer and to reach "E" before the tank is dry. This is mostly psychological; by making the needle linger longer near the upper end of the range they can make people perceive the vehicles as more efficient than they really are.

In some cars, the gauges actually are linear. In my experience this is usually cars from Northern Europe. Volvo's typically mark the gauge with gallons remaining (the gauge in my S70, for example, has numbers for 18, 12, 6, and 2 gallons remaining, with linear tick marks every 2 gallons, with the "E" coming at the point where 1 gallon would have been marked) and the Porsche's I've had have all read linearly as well, with the "full" mark being marked as "4/4" and half tank as "2/4." Most of the time the "manipulative" gauges I've seen are in Asian and North American cars, but I'm just going on my experience with my own and my family's vehicles here.

ZV

 

[JCH13]

I think the needle may stay on "full" for a little bit because whatever volume of fuel that is in the filler neck needs to be consumed before the level tank itself starts to change. This can be a considerable amount of fuel.

Intermediate levels change at different rates because fuel tanks generally have a non-constant cross section that is difficult to calibrate to.

 

[mikeymikec]

My Mazda is a bit irritating in this respect - filling the tank right up results in the needle going about 2mm over full, the half-way mark is at about 220 miles, then the warning light comes on (and can go off again depending on which way the wind is blowing) at about 350 miles. I'm not sure whether because the fuel needle goes 2mm over the full mark, if that means 'true empty' is 2mm above the empty mark. I also don't like the fact that the manual doesn't mention how many litres ought to be left in the tank when the warning light comes on*.

For all the things I disliked about my last Volkswagen, both it and its VW predecessor didn't do the weird warning light intermittency.

* - again, unlike both VWs I've owned.

 

[TehMac]

Actually, it doesn't. It's a valiant attempt and it seems like common sense, but it's not the actual reason.

The gauges in most cars are intentionally designed to stay at the "full" mark longer and to reach "E" before the tank is dry. This is mostly psychological; by making the needle linger longer near the upper end of the range they can make people perceive the vehicles as more efficient than they really are.

their explanation seemed fairly logical--the gauge is connected to a 'floater' which is hardly the most accurate form of measurement.

 

[waggy]

this is why i like having the "computer" that says how many miles until empty. it gives me a better idea on how much fuel is really in the tank.

also i hit the trip function every fill up. i know how many miles i got before i need to fill up.

 

[rommelrommel]

this is why i like having the "computer" that says how many miles until empty. it gives me a better idea on how much fuel is really in the tank.

also i hit the trip function every fill up. i know how many miles i got before i need to fill up.

True, but most just take your average estimated mileage and what the fuel gauge says is in the tank. I've had some pretty good ones (current 300C SRT8 is very good) and some that were basically nonsense.

 

[Zenmervolt]

their explanation seemed fairly logical--the gauge is connected to a 'floater' which is hardly the most accurate form of measurement.

Their explanation is simple, neat, and wrong. That's all there is to it. As logical as it is, the size of the float is simply not large enough to create any meaningful amount of uncertainty in the reading.

The gauge is connected to the same thing that feeds the ECU information for its display of "miles remaining." The float is quite accurate for determining the level of gasoline remaining as long as the results are properly calibrated. Provided that the tank's shape is known, it's almost trivially simple for an ECU to translate the float's position along its travel into the amount of fuel remaining in the tank and use that calculated value to drive the gauge.

The mere fact that there are millions of cars out there with linear fuel gauges proves this.

On vehicles which do not have linear fuel gauges, the non-linearity is generally programmed into the system for driving the gauge. This is similar to the way that many modern OEM gauges (like engine temperature, battery voltage, and oil pressure) are programmed. Gauges like engine temperature, voltage, and oil pressure are programmed to stay in the middle once operating norms have been reached and then only fluctuate if there is a massive change; that's why the factory "oil pressure" gauge on my old '06 Mustang didn't budge if you revved the engine.

The explanation offered by the cracked.com article may have been true a very long time ago when gauges were much simpler, and it may even still be a very tiny factor today, but it's not even close to being the main player in the way that modern fuel gauges display tank level.

ZV

 

[CraigRT]

My take:

Gas tanks are shaped weird to fit the underbody.
As it tapers down it changes the gauge's readout.

 

[DisgruntledVirus]

this is why i like having the "computer" that says how many miles until empty. it gives me a better idea on how much fuel is really in the tank.

also i hit the trip function every fill up. i know how many miles i got before i need to fill up.

I had a Ford Focus that told me "40 miles to empty" as it ran out of gas... Fucking a mile from my house and 1.5 miles from the gas station...

 

[phucheneh]

I had a Ford Focus that told me "40 miles to empty" as it ran out of gas... Fucking a mile from my house and 1.5 miles from the gas station...

That's very surprising. I was going to state those 'trip computers' are just an overly-elaborate digital version of the old analog gauge. They still generally say '0 miles' with a couple gallons of reserve in the tank. And the 'miles remaining' often decrease a lot more slowly at the top of the tank. Conversely, I've seen cars where the the last, say, maybe 30-40 miles go really quickly. They really do behave a lot like an old school directly analog-driven (as in pre-EFI) gauge.

The Cracked thing might not be that far off, but I don't think it's coincidence or simply laziness in design. The reason for an empty reserve is obvious; as far as seeing full when you're still a couple gallons shy...I would wager the evap system is a large reason for not caring about an accurate 'full' level. If cars started only reading full if you pumped all the way to the auto shutoff, I'm sure overfilling (and subsequent evap system malfunction/damage) would be more widespread.

Basically, I think we could have more accurate gauges, but it would probably create problems through the consumer's misunderstanding and/or old habits.

 

[Zenmervolt]

That's very surprising. I was going to state those 'trip computers' are just an overly-elaborate digital version of the old analog gauge. They still generally say '0 miles' with a couple gallons of reserve in the tank. And the 'miles remaining' often decrease a lot more slowly at the top of the tank. Conversely, I've seen cars where the the last, say, maybe 30-40 miles go really quickly. They really do behave a lot like an old school directly analog-driven (as in pre-EFI) gauge.

I still think it depends more on the particular car and on the specific design parameters chosen by the engineering team.

I've driven several Fords and the "miles to empty" displays did run down to "0" before the tank actually ran dry, but I didn't notice any big non-linearity in their actual tracking of miles. This is similarly true of the Hondas I've driven with such displays. But it's possible I wasn't paying meticulous attention.

In my Volvo though, the display varies wildly throughout the entire tank. As best I can determine, the "miles to empty" display on the Volvo is based on a relatively short running interval of the car's recent average mpg. My daily commute is about 20 miles, mostly uphill on the way in and mostly downhill on the way home. The "miles to empty" display will show me losing about 30 miles of range on the way in to work, and then only show me losing about 10 miles of range on the way home because of the way its predictions change based on the running average. I've even seen it start showing extra range during a long downhill section. If I've been traveling on a freeway for a long time and then get off onto surface streets for a couple miles, the "miles to empty" display will drop like a stone because of the sudden transition to city driving.

It all depends on how much smoothing the company decided to apply to the running average mpg value used to compute the miles to empty result.

ZV