Fuel Gauge and Miles left in Tank

Grumpy_chap

DE_612183 said:

At the moment 485 has stayed that way for about 100 miles, then drops and works ok.

But I have to remember to set the trip when I fill up in case it stops altogether!

It's under warranty, so ask the Dealer, but my car is rather similar to that.

I fill to the brim, so the fuel gauge needle goes above the top white line. 
I guess the top white line is calibrated to "10 gallons" or something.
While I am driving around at above the "10 gallons" calibration, the computer only sees the maximum "10 gallons" until the fuel level goes lower into the measuring range.
Then the forecast starts to change as fuel is used and efficiency (driving style) goes up or down.

My car is a 2007 Focus, so nothing like a "VW-characteristic".  In a way, the problem is solved in my car as the LCD display stopped working so I just see blank for anything.

Remaining range is not that important when you have a lot of fuel - you know you have a good distance ahead and no need to fill up.  It's only as you see lower range, say under 100 miles remains, and the motorway sign says "services 35 miles, 80 miles" that you need to be making a decision based on the remaining range.

It's very rare to see people expressing range anxiety when they have ICE - just wait for EV.

diystarter7

It varies even on the same make/model of car as this is subject to ones driving style, traffic, weather conditions, road layouts, etc, etc.

I've looked on my Ford at times it goes up and down ie the dash showing MPG util empty and this is because of how we drive.
Best to get the car filled up soon as the light comes on but I guess I'm good for another 15 town driving and a bit more on straight runs in warm weather. Weather does affect MPG as well as having the air con on etc and the way you drive.

diystarter7

DE_612183 said:

Hi, I've got a fairly new car ( less than a year old ), I have a problem that the fuel gauge is got reducing with usage.
I reset the "trip" every time I fill up, and usually after a while the correct value is displayed - but I wonder what is happening.
I know in old cars there used to be a float, that would measure the tank contents - which was why if you parked on a hill the tank could show as full or empty depending on which way you were pointing!

Any ideas on what could be wrong and if there is an easy fix?

Thanks

I've noted that on my car and dads car especially if there is around a quarter of a tank but soon as you are almost level shows true value. I don't think anything is wrong with your car but as its covered by warranty, ask and I guess they will tell you the same.

Herzlos

It looks like it's still got a standard fuel gauge in the right - why worry about the trip mileage instead of just refilling it when you get to 1/2 or 1/4 full?

The numeric range display is often referred to as a guess-o-meter for good reason; it's using your previous driving economy to guess at the remaining range.

Ibrahim5

I decided my fuel gauge must be inaccurate based on size of tank. So I drove it with the fuel gauge showing empty but watching the mileage. It conked out on a hill. It was a bit embarrassing telling the breakdown operator that I might have run out of fuel. She asked what the gauge showed and I had to say completely empty. Didn't take long for a man to come and put some 'free' fuel in.

Manxman_in_exile

OP - I don't think you're talking about the "fuel gauge".  Don't you mean the digital readout?  (The particular value is called "Distance to Empty" in our Mondeo).

Your car is constantly measuring the distance travelled and the fuel used, and uses this to calculate your average mpg since you last reset the figures.  Each time you fill up it knows how much fuel you've put in and how much was left in the tank, and it uses these two figures together with your average mpg to calculate the Distance to empty.

The Distance to Empty figure is a real time calculation and is constantly being updated.  If your driving style and journey types are pretty constant, you should see the figure decrease gradually as you use up fuel.  But if you change your driving style and/or journey type so as to affect your average mpg, the Distance to Empty figure may behave a bit strangely.

We mostly use our car for relatively short and low average mpg journeys around town.  If we go for a long motorway journey (or something similar with a higher than usual average mpg) it's not uncommon to see the Distance to Empty figure increase rather than decrease.  

I've gone for a 100 mile motorway journey and the distance to empty figure hasn't changed at all - it may even have gone up.  And I often play around with different driving styles to see by how much I can increase the figure.

So if that might explain what you are seeing, there may be no need to ask the dealer.

If you've got a proper "fuel gauge" (a semi-circular dial with a needle pointer) that's probably a more reliable indicator of when you need to fill up rather than relying on the spurious precision and accuracy of a digital readout.

diystarter7

@Manxman_in_exile

The OP is talikg about two items.

The dig readouts and when the car is parked on an incline and ref to the fuel gauge.
Please see my previous post if you wish.

DE_612183

yes - the fuel gauge says - full tank
the miles to empty shows - full tank ( or equates to it )

But neither are right - so what I was trying to work out is "how" the fuel in the tank is measured?

It used to be a float - but is it more electronic now?

Username03725

DE_612183 said:

It used to be a float - but is it more electronic now?

It's not a float sitting on top of all the fuel in the tank swishing about as the fuel moves. It's a narrow vertical tube that fuel easily flows in and out of, but doesn't fluctuate as much, and the sensor floats on top of the fuel in that tube. It also doesn't go right to the very top of the tank + filler neck, so when you fill up the tank is holding more fuel than the sensor can register; it's above its 100% and will be till the level drops to anything below the top of the sensor tube. In that time the on-board computer is getting a constant Full signal, so calculates range based on a full tank. You can prove this. Run it as close to empty as you dare then brim the tank and compare the amount you've put in against the car's listed capacity. A shiny English pounds says you can get more in than the handbook says you can.

Range is a moveable feast anyway. It was a running joke with our kids when they were younger that we'd set off on a long run with a range showing of e.g. 165 miles, and 100 miles later it would be showing 205. Range is continually calculated based on mpg to date, and running round town & school drops etc gives a much lower mpg than pootling up the M1 at 55 through the endless roadworks that we had then. A bit of 32mpg followed by a long spell at around 70mpg (diesel) throws the calculation out a bit, till it settles down. Resetting the trip & MPG to zero overcomes the anomaly, if you're that bothered. I never was.

DE_612183

Update - car has gone 78 miles ( as per trip recorder ), but tank level is still showing as full ( 8 bars ), but the Mile to next fill has gone down by 20 miles!