Fuel gauge confusion

Morbius

Morbius

Morbius

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Joined
16 Jul 2008
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2,241
Hi all.

My fuel gauge has always confused me. I have a YBR125 which is supposed to have a 13 litre capacity.

The other day I got home and the needle was just above the red section that comes before empty. I took my 5 litre fuel can to a well known supermarket's petrol pumps and filled it. I then went back home and emptied all 5 litres into the bike. The needle then went to full. I was expecting it to need another 8 litres to get to full.

Now, today I took the bike to work, which is 25 miles each way, so 50 in total. When I got home in the evening, the needle was at halfway. Now, if we assume that means half of 5 litres then that equates to roughly 90mpg, which is par for the bike.

Are all fuel gauges this inaccurate? I suspect probably not. If I had to guess, I reckon that something might be up with the fuel sender. Because I'm still fairly new to motorbikes, I'd appreciate any advice or suggestions.
 
I've found bike fuel gauges to be soo inaccurate that they are worth ignoring.
I measure fuel available based on knowing what my maximum range is, and leaving a healthy margin before brimming it.
I reset my trip counter each time I refuel.
 
My fuel gauge reads 'full' from the moment I fill it to about 25 miles short of running out (I get ~200 miles to the tank at the moment). Then it notices I might be running out.

In short, ignore. On my 125, I went of the odometer. 200 miles = fill up.
 
As a general rule, "full" on the gauge is not necessarily actually full. In various cars and bikes I've found that you can drive/ride around with the gauge showing full for quite a long time. Just use it as an indication of when to fill up, not as a precise measurement of how much is in the tank.
 
Mine wasn't too bad until last weekend. Digital display with 5 bars for full. Since Sunday tho, when I brim it I only get 4 bars. I assume the sensor has slipped or sumink. Still under warranty so I dunno whether to bother getting it looked at.

But like others have said. I tend to go by mileage anyway. Reset the counter with each fill up. The bike has 2 trip counters I can use, which is handy. I use trip A for fueling and trip B for last service :)
 
The fuel gauge is a general indication only, it gives you a rough idea when you need to fill up. Very few fuel gauges are linear, and especially so on bikes which tend to have odd shaped fuel tanks. The red line might mean you have a few hundred yards left, or it may mean you have another 30-40 miles or more left, you have to learn the traits of each bike. This also applies to cars, where again different models have different traits.

When you put the 5 litres in, did you actually look into the filler and check the fuel level?
 
I have owned 2 bikes. A 125 with simply a reserve tap so you just used trip counter to measure trips and you begin to know when to fill up.

Then my GSXR 600 I seem to recall had a low fuel warning indicator. I think it used to be able to go 20 odd miles on low fuel light. Again measured on the trip counter really.

Even my car fuel guages are pretty poor and always lying.
 
OK, I think I've got to the bottom of it. I took the bike to said supermarket forecourt to fill up and kept going until I could see for certain that the tank was full. I now know that full is a small amount past the full mark, so there must be quite a healthy amount of fuel remaining when the needle gets near to empty. That's no bad thing really.
 
On the YBR's it says way over full for about 50 miles, then it starts dropping rapidly. Mine ran on empty (so it said) for nearly a week. I got 260 miles out of that tank.
 
My Z750 was awful for this. There were I think 5 bars on the gauge.

It'd go like this:

Five bars
Five bars
Five bars
Five bars
Five bars
Five bars
Five bars
Five bars
Five bars
Five bars
Four bars
Three bars no wait two
Fuel light
 
I actually did some work on motorbike fuel gauges not so long ago, it's not till you look into it you realise what a difficult thing it is to measure. Constantly sloshing fuel requires a lot of averaging to get a realistic reading, and it doesn't help that it's near impossible to measure to the very top or very bottom of the tank (in fact most bikes measure less than 70% of the range!). The biggest issue though is predicting the riders filling behaviour, some people push the nozzle right in, some drip feed it to the very top, and some even drill holes in the tank to fill the air gaps in the top (which is there for a reason I might add, fuel expands when warm, without it yer fuel comes ****ing out the overflow on a hot day). All this confuses makes it very easy to confuse the ECU.

Conclusion is don't trust the fuel gauge (on motorbikes, cars are a different matter), it should only be used as a rough guide, instead use your trip meter to gauge how much you've got left.
 
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