Petrol Meter Gauge Faulty?

[Annastesia]

Hello,

I'm hoping I'm just over thinking here...

My meter guage resembles the one in my attachment.

Since buying the car 2 years ago, whenever I fill up my tank the gauge shows as full (20 bars).
I can travel up to 50/60 miles before the first bar goes (to 19). And then the bars steadily decrease.
Overall, it usually works out about 25 miles per bar.

But today when I filled it up the bars started reducing straight away.
It went to 19 bars after 23 miles, 16 bars after 100 miles - which is accurate, but it's just that it didn't stay full on 20 bars for that first 40/50 miles, and steady decrease.

Although the miles per bar is accurate, it's just that this is the first time it's started decreasing from the off.

I don't know if I'm worrying about nothing or if there's something that may be cause for concern.

[ColinB]

The reason the gauge doesn't move for the first few miles after filling up is because it uses a float sensor in the tank itself, so that won't register any usage until the fuel in the filler neck has been used and the level in the tank has started to reduce. It seems likely that when you last filled up, you didn't fill it right up to the brim, so the neck wasn't completely full. Try filling again, this time taking care to fill right to the brim.

[Annastesia]

Thanks very much Colin B!

I will definitely do this next time and keep a check, I'm just glad it doesn't sound like a bad thing.

I didn't know it used a floating device, but that must be why I occasionally see it lose a bar, go up again for a while and  then back down.

It's making sense now 

[Kremmen]

In most cars the fuel tank is a standard 'box' normally partly located under the rear seats

With the Honda magic seats this is impossible so the fuel tank is long, wide and thin and is under all seats. This makes reading the actual fuel level more difficult.

Another downside of the 'under all seats' is that if you only put in a small amount of fuel, not the case here, the gauge and tank range often doesn't register it

[Twinkie]

In most cars the fuel tank is a standard 'box' normally partly located under the rear seats

With the Honda magic seats this is impossible so the fuel tank is long, wide and thin and is under all seats. This makes reading the actual fuel level more difficult.

Another downside of the 'under all seats' is that if you only put in a small amount of fuel, not the case here, the gauge and tank range often doesn't register it

Thank you very interesting that explains a lot.

[Lord Voltermore]

Some forecourt petrol pumps deliver fuel faster  than others. A faster than normal pump might have delivered fuel faster than the cars filler neck could cope with it. Particularly near the end.    The neck  gets  temporarily full of fuel, and the pump 'kicks back' making it appear the car is already full.  But by  waiting a few seconds for the petrol to drain down you can often get another 2 -3 litres in. With a slower pump you typically add this extra 2-3 litres as a matter of course,witout realisng it  because the pump doesnt kick back early.  Thus for your first bar you use the extra 2 -3 litres from the filler neck before the tank even starts to drop.

Some cars are more  prone to this false kick back than others and often  have to be filled extra slowly , even on relatively slow pumps. They can be a nuisance . It sounds like your car doesnt normally do this, except on a very fast pump. 

[embee]

There is always "hysteresis" built into fuel gauges, usually electronically, so it doesn't keep flipping back and forth between readings ("bars") as it passes a certain level. However you often find, depending on the model, that it can go back up a bar if for example you go uphill for an extended period (or downhill if that's the way the float works). Conversely it can make the apparent level drop a bar unexpectedly early if you go onto a gradient.
As described by others above, there are practical aspects to the system, including the shape of the tank, which can affect how precise or stable the readings might be on any given day. As long as your overall fuel usage is reasonably consistent then fuel gauge accuracy is just something to live with.
Sounds like hopefully the OP's gauge is behaving OK if a little variable.