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Is there any reason to use a discrete representation (like number of stars) for continuous quantities (like a progress bar)? This is assuming we don't want to show an exact number. (Yes I understand that values in computers are not "truly" continuous, but for the purposes of this question, we'll say they're close enough)

This question stems from my car and its stress-inducing gas gauge. This is what it looks like when full:

Digital Dashboard - full

The gas gauge is made up of 6 stacked bars, which are all lit when the gas tank is full. This is what it looks like when close to empty:

Digital Dashboard - empty

Now here's where it gets worse. That bottom bar flashes about once per second any time the gauge is down to one bar. In other words, the last fuel bar flashes for ~17% of a full tank. Since I mostly drive full-to-empty, the car seems to be telling me its out of gas 17% of the time I'm driving it (and distracting me with a blinking light)

Now, some quick math to show how stress-inducing this is:

The tank size is a little over 6 gallons. The fuel efficiency though, is ~45mpg average (why I like the car). This means that on a full tank, I have a range of roughly 270 miles.

The last bar on the gas gauge therefore represents ~45 miles of range. To put that in perspective, with commute and all, I drive about 20 miles a day. This means that I can actually drive for 2 full days while the car screams that its about to run dry. When I get in my car in the morning and the bar is flashing, I get to play the fun game of "Can I actually make it to work today?".

All ranting aside, it seems that the obvious solution would be to have a closer-to-continuous gauge. Even if it was just 10 bars instead of six, that gives the user a much closer idea of the actual value.

So, is there any UX reason for doing something like this, or is this just bad design?

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    In this specific case, LCD display technology requires discrete elements. Perhaps they could have put more elements for steps at a higher cost, but it would have still been discrete. – dbkk Jan 7 at 18:57
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    what dbkk says. This is an example of the choice of display technology affecting the ux and what can be done, it wasn't a specific ux decision to chunk the information. – mgraham Jan 7 at 19:32
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Adding to Chritian's answer,

UX improves over time

The most recent (and high-end) vehicles also show real-time mileage and a rough estimate of how long before you should fill up.

Other reasons

  • Cars are mass produced and not all the components are tailor-made. The digital display in your car might be being used in multiple vehicles across countries.

  • It's not always possible make indicators precise. The more granularity you add, the higher the chance of discrepancies. Battery indicators in phones have suffered this many a times.

  • Definition of low fuel can be subjective. The fuel mileage of a vehicle isn't a constant value and can have many factors that make it highly variable. If the fuel gauge were to show measurement in percentage, you'd be in a sticky situation if you thought 10% is enough to get you to the next refueling point but it hastily drops to 0% due to some reason.


Also, I'd like to point out something here. You might think that roughly 45 miles worth of gas isn't urgent enough for your car to warn you about low fuel but imagine if you were on a long drive and suddenly saw the Low Fuel indicator. You'd be glad that there's enough headroom for refueling.

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So, is there any UX reason for doing something like this, or is this just bad design?

There is a definite reason for this in my eyes, the learnability of interface and the predictability of the resulting behaviour.

If I have a phone / car etc with "chunks" of fuel, I can begin to learn how long a chunk lasts, "I'm on one bar on my phone" that means I have ~4 hours left. I'm on the "warning level" on the fuel tank, that will get me 50 miles.

A continuous gauge can be helpful if precise, but it would be easy to go from say 35% batter to 15% battery without perhaps noticing that I'm in the last quarter. I personally find this on my own phone, the entire battery indicator is something like 20 pixels high so a tiny 1px visual change can indicate a 5% battery loss.

The analogy I'd use would be boiling a frog, if the water increases gradually the frog doesn't jump out. Likewise if my fuel goes down gradually or my battery it's harder to notice the difference.

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This is not the most fortunate design. Usually with mecanical indicators when the indicator was showing empty you would still have around 7 liters of gas. They do this for two reasons. One to encourage you to fill up your tank so you won't get stranded somewhere on the highway and the second one is to not use the gas at the bottom of the tank because that is where the dirt and impurities sit and if your engine would start sucking the impurities chances are it will damage it. So this is why they point out the urgency.

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