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Hi everyone,

I had my Mazda3 2014 for a little over a year now. It always seemed to me that the fuel gauge is over-cautious by a generous margin, but only last week I put this assumption to the test. I ran until there was one little line left on the fuel gauge and the Range estimate was 10 kilometers (6 miles).

Hence, according to the dash indicators, I should have had roughly 2 liters (half a gallon) of fuel left in the tank. Since the tank capacity is 50 Lit (13 Gal) I thought that, in order to fill it, I would need roughly 48 Lit (50 - 2) or 12.5 Gal (13 - 0.5) of fuel.

But the pump stopped at 38 Lit (10 Gal). The margin is then 10 Lit (or 2.5 Gal) - which is huge and leads me to believe that there's probably something wrong with the fuel gauge.

How is your fuel gauge behaving? Has anyone experienced something similar?

Thanks in advance for any answers.
 

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mine behaves exactly the same. I've driven until range is 0 and couldn't pump more than 10.5 gallons. So I think there is a "reserve tank" of 2.5 gallons, and the fuel gauge is programmed to go to empty with 2.5 gallons in order to maintain a certain fuel pressure for our high compression ratio engines (I think I read that on here a long time ago)
 

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Modern cars are engineered to be idiot proof. Driving with such low fuel levels is foolish, and the reserve tank has nothing to do with high compression. It's so people testing their assumptions don't run their tank dry.
 

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Yep, this is pretty normal and not just on the Mazda cars these days. Even my 2010 Hyundai was like this. Once you actually hit empty, there's still a little bit left. That way you stop and get gas before it's actually empty. As others have said, it's to "idiot proof" the car.
 
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It's a feature, not a malfunction.

And there's a reason it exists too, and it's not to abuse it, it's to cool your fuel pump that's physically within the fuel tank.
You start running on "0" miles left, and you'll burn up your fuel tank.

929s used to have a not so great feature that was the, for the lack of a better term poor valving on the fuel tank, and if you drove far enough, the fuel tank would implode.

Yes.
Implode.


So, do yourself a favor, if you're at 1/4 tank of gas, go fill up. Because all you're doing beyond that is causing damage, and running on empty can become very expensive, very fast.
Especially if any valves are stuck or blocked, and you blow up a fuel pump, or too damage the structure of the fuel tank, and have to get the car towed, and have them drop the fuel tank and check all of the lines.


Yes, I admit I like to scare people as much as possible whenever this subject comes up, but it's deserved.
Just don't do it.
 

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So, do yourself a favor, if you're at 1/4 tank of gas, go fill up. Because all you're doing beyond that is causing damage, and running on empty can become very expensive, very fast.
Especially if any valves are stuck or blocked, and you blow up a fuel pump, or too damage the structure of the fuel tank, and have to get the car towed, and have them drop the fuel tank and check all of the lines.
Look, which is it? Is the guage idiot proof so "0" is a safe number to fill up at (given that there are still GALLONS in the tank), or should you fill up at 1/4 tank? Why not be safe and do a half a tank? Why not mount a 500-gallon drum of fuel to the roof and make sure it's always half full?
 

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Look, which is it? Is the guage idiot proof so "0" is a safe number to fill up at (given that there are still GALLONS in the tank), or should you fill up at 1/4 tank? Why not be safe and do a half a tank? Why not mount a 500-gallon drum of fuel to the roof and make sure it's always half full?
Because there are people like you that just want to prod silly statements when the reality is it doesn't matter, it's just a recommendation.

Do what you want to your car, it isn't mine, but if anyone runs out of fuel and has an issue, I will point my finger and said I told you so.


Just because you can, doesn't mean it's a good idea.
 

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Discussion Starter #11
Thanks, everybody, for your answers! Knowing that this is normal behaviour will at least save me a trip to the dealership.

Now, onto the topic of why this is being done at all... The owner's manuel does indeed recommend keeping the tank over 25% full, and that's fine by me. I also understand the need for margins in order to "idiot-proof" the system - that's also fine.

But I find the margin a bit too generous. Given the way the gauge is built, it means that the tank is ALWAYS at least 20% full - since at that point the dashboard shows "0" miles left. In theory the gauge should run from 0% to 100% but in reality it only runs from 20% to 100%. Hence, when one is looking at a fuel gauge that shows 25% full, it actually means that the fuel tank is 45% full! Computing the car's mileage by myself would yield wrong results then, since I'm only using 40L from full to empty instead of 50L - a pretty big difference.

I personally don't really like to drive with a full gas of tank everywhere, as it adds weight to the car. Negligible, I know, but still! A full tank is roughly 80lbs. Since I do a lot of short-distance driving, why carry that extra weight with me?
 

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There is no 100% accurate way of reading fuel mileage though or knowing how much is left etc. anyways; which is why if you've ever heard anyone mention that you should always go to the same gas pump when you can, do. Because otherwise the only way to accurately measure is to have a system that is guaranteed to be visible and empty out entirely and have a gravity fed system...

In short, gas mileage is relative. It's not perfect, it just works "good enough" for most people.

As far as the weight goes, it is negligible, if you're serious about it, sure, go to 1/4 tank and fill up to 3/4, but then... why? What's the point? What are you honestly saving? The weight added isn't altering your MPG by more than half an MPG for a short while at most.

As for the car computing the wrong mileage, that is not true, as that is not the way it actually measures (if you're talking about average and current MPG), that it physically calculates through the ECU how much fuel it's using to come up with those numbers.
Miles remaining, is yes, partially based on that 20% remaining number, BUT, remember that is there for a reason; it's NOT to be confused with fuel remaining in tank, it's fuel remaining before you SHOULD be filling up. Because as mentioned earlier; a bone dry fuel tank is dangerous, and can lead to a number of broken parts that will need replacing.

But the miles remaining is actually based on your "recent" average MPG. Even if you reset your average MPG, it still bases that number off I suppose X days or something related to your more recent driving style.
 

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Driving around with a full tank really isn't going to change much in the way of fuel mileage and wear on your car. We are talking fractions of a percent. You would get greater gains by going with lightweight wheels than less fuel.
 
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