or, you could estimate it fairly accurately using the following formula:
(fuel tank capacity - gallons needed to fill tank when gas light turns on) * (average miles per gallon)
I have about 4 gallons left when the light turns on. 2 gallons left when the needle dips below the "empty" line. The emptiest my tank has ever been is 0.5 gallons left.
For what it's worth, my car's fuel gauge appears to do exactly this—no change from full for the first 60-100 miles, and about 2.5 gallons left on "empty." The first quarter tank lasts a good deal longer than the third, at least according to the gauge.
You'd think you wouldn't want to do this with a range estimation because the error's more obvious, but I'm also sure they don't want to sandbag its estimate from a full charge because people will be wondering why they're not seeing the advertised range. I'm sure they do want the car to be able to keep going a little past empty, though, and those spare miles have to come from somewhere.
How many times do you need to make that calculation?
I know how far I can go on one tank and how much it costs to fill - everything that follows is trivial arithmetic.
My 2010 Impreza behaves in much the same way. Light comes on at a quarter tank, and when fuel gauge needle bottoms out you are out of fuel. It's not exactly where the needle points to empty though, the empty line is pretty wide. It's empty at the point where the indicator needle reaches the bottom of that line. There isn't any sort of peg, so you can't really be sure of how much fuel you have left, but at the same time the light motivates me to fill my car way before that point so I don't have to worry.
A typical person knows how many gallons their gas tank can hold, and thus how far they can go without needing to refuel. For day to day driving, it doesn't really matter. For long road trips, it can be essential if you're on an extended barren interstate highway.
My first car, we couldn't keep the gas gauge working reliably, so I had to reckon the mileage from the odometer to keep track of whether it needed to be filled. At least it did have dual gas tanks, so in the worst case, you'd notice the engine was chugging and trying to die, and have to flip the switch to the other tank in time.
Filling up both tanks kind of sucked, back when gas was $3.50-4.00/gallon.
> In a Mercedes [...] the tank is really empty as soon as the needle hits 0
Not in mine (in Germany, recent C class). It depends a bit on whether you define "reserve level" as 0 or not. On the one hand, the needle does go below reserve level all the way to physical zero, but from a UI point of view, reserve level itself is clearly what they want you to see as "running on empty". And that's what they mean in this article, too.
In my car, it actually shows you in the dashboard how much range you got left based on your current driving style, too. I have a medium-power Diesel engine, so that works out to about 40 miles remaining once the warning light goes on, and that's pretty accurate. After half of that is used up, the dashboard stops showing the estimate and instead begins to flash a "fill up immediately!" message.
> It's also possible that it keeps a buffer of ~1 gallon of gas that it never tells us about, when quoting the total range, and it eats into this buffer without telling us.
This is quite likely. My car supposedly has an 11-gallon gas tank, and if I refuel right when I hit 0 miles remaining then it only takes 10 gallons of fuel to fill the tank.
I'll do you one better: mine has a 45 litre tank, light goes on at 5 litres left, gauge stops indicating at half of that, but there's an unmentioned anywhere buffer of 5 litres, which is there so that the fuel pump doesn't overheat or pick up any contaminants at the very bottom the tank.
I only know this because a motoring journalist filmed himself riding the same model dry.
What I do is to ignore the gas gauge completely. I keep track of the miles driven. My driving is fairly consistent, and I know the rated capacity of my fuel tank, so I can fairly easily determine when I should fill up based on how many miles I have driven.
I have a 2014 Nissan Altima. It gets better gas mileage than any vehicle I've ever driven, other than motorcycles. On long highway/interstate trips, I get 550-600 miles on a tank of gas (the tank holds about 12 or 13 gallons, if memory serves).
My girlfriend has been driving it for the last year or so and the only times I drive it anymore is to take it in for an oil change or maintenance, but I often check the statistics and it averages 34 mpg overall (most of her driving is highway). The "low fuel" indicator in it also comes on with somewhere around 100 miles remaining.
I did drive it far enough once without stopping for gas that the "miles left" indicator read "---". I was about 10 minutes from home and figured I'd chance it (there's a gas station at the intersection where I turn off the highway to come home). My thought was that if I had designed the "countdown indicator", I would be in a bit of a buffer so that the car doesn't actually die going down the road when it hits zero, so I was certain I still had enough gas left to make it.
My truck (large 2013 Dodge Ram) averages right about half of that. Its "low fuel" indicator comes on with about 50 miles remaining. The Harley gets around 50 mpg (and offers to find the nearest gas stations when there is ~35 miles remaining).
The specific amount of fuel in the tank is a number that I would think most people wouldn't really care about.
Some cars will give you calculated "miles/km till empty" based on average mpg calculations, which is arguably at least a semi-useful metric. My car gives me a vague gauge until I get close to zero, and then it gives me a marginally more accurate countdown from 1.0 gal to zero.
I think the reason that most fuel gauges are so vague is that fuel levels rise and fall pretty erratically, which would make a very specific gauge give you bad information at least as often as it would give you good. Fuel levels are usually measured with some type of float, and depending on if you were travelling or parked on an incline the float could register a fairly large swing.
In an old Honda I drive once in a while I could see the fuel gauge swing more than 1/8 of a tank depending on what direction I parked it on my sloping driveway (nose down or nose up).
A quick scour of Google seems to come up with about 0.2 gallons per hour (about 900ml) for an idling engine, so assuming you keep your engine running (and discounting extra fuel used moving forward in the line), let's say you have a 16 gallon tank, you'll be able to run the engine for about 80 hours before stalling. Plus or minus 20% bearing in mind size of engine, tank size, ratio of time spent idle vs. moving, how much fuel you start with (you wouldn't be at a fuel station with a full tank, I'd imagine?)
80 hours is a lot less than I thought it would be.
I have about 4 gallons left when the light turns on. 2 gallons left when the needle dips below the "empty" line. The emptiest my tank has ever been is 0.5 gallons left.
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