Fuel tank size, warning light threshold.. and more emissions cheating?

[kalniel]

1) Car as per sig.. should be a 50l fuel tank according to the manual, and yellow warning light should come on around 7l to go, right? So I'm a bit confused when I filled up today - managed to get about 45l in, yet I'd seen no warning light appear. Trip computer was indicating 70miles left (but then it was indicating the same when I started the journey, some 25 miles earlier). Faulty sensor or just a bigger fuel tank than indicated (by at least 2-3l)?

2) The other pattern I've spotted having strayed into low fuel regions sort of regularly, is I get a massive increase in economy during the last 10l or so in the tank. Granted, I might drive a little more smoothly when I'm (worried about) cutting it fine, but it's very little difference given I try and drive economically at other times anyway, but where on the same journey/time of year/day etc. I'd normally average an indicated low 50s MPG, when on low fuel this leaps to an indicated low 60s.

Given I calculate whole tank MPGs, I can't determine if this is just an even less accurate MPG readout than usual (it's already wildly over-generous as is) or if there really is an increase in efficiency when on low fuel. If so, why? It can't be weight, since I don't see any significant difference in MPG when I'm 1up verses carrying a 70kg passenger. Might emissions regulations be reduced in low-fuel states to boost economy?

 

[cuke2u]

My warning light comes on at around 50 miles left, I think its around 5L, but it isn't determined just by the fuel remaining as it is also by mpg. Are you determining mpg by average or by instant and the last paragraph is just rubbish in reference to any emissions cheating...

 

[kalniel]

My warning light comes on at around 50 miles left, I think its around 5L, but it isn't determined just by the fuel remaining as it is also by mpg. Are you determining mpg by average or by instant and the last paragraph is just rubbish in reference to any emissions cheating...

The MPG figures I mention are journey average, which in my case is at the end of 25+ miles. Same traffic, weather, temperature etc. They are quite wildly over-optimistic compared to a nearly whole tank miles/fuel used figure. If affected by MPG then it's different from the reserve light on previous A3s which was a fixed volume remaining (7l).

If rubbish, then any other explanations? Remember emissions reduction is turned off at low temps, why mightn't it also be turned off in low-fuel conditions?

 

[richinsoton]

You've also forgotten about XX numbers of litres in the filler neck if you've filled it until brimmed. I wouldn't be surprised if that held another 3 litres at a guess, so I wouldn't be surprised at getting 45 litres into your car.

 

[kalniel]

You've also forgotten about XX numbers of litres in the filler neck if you've filled it until brimmed. I wouldn't be surprised if that held another 3 litres at a guess, so I wouldn't be surprised at getting 45 litres into your car.

That likely explains the 45l fill then, though surprised they don't count that as part of the fuel tank specifications since I'm just filling till nozzle cut-off as recommended. I had always thought 50l was a smidge on the small side but 52/53 would look better

 

[Terminator x]

I run mine low (<30 miles left according to trip computer) and always manage to get about 49 litres in what the Spec says is a 55L tank; about right I guess as 5-6 litres left is about that in miles to go. I only fill until it clicks rather than fill the neck with fuel too!

TX.

 

[Daveotto]

I run mine low (<30 miles left according to trip computer) and always manage to get about 49 litres in what the Spec says is a 55L tank; about right I guess as 5-6 litres left is about that in miles to go. I only fill until it clicks rather than fill the neck with fuel too!

TX.

That's exactly what I do, run to less than 30 to empty and top up and typically get 49-50 litres in the slightly larger 55l tank you get in the S/RS3

In my last 4 A3's (1 x 1.4TFSI, 1 x 2.0TDi, 2 x S3's) they have all behaved the same ...fuel light comes on at around 30-40 until empty and fills up with approx. 4-5 litres less than tank nominal capacity. On a couple of occasions I have drive 10-15 miles when on 0 to empty in my latest s3 and have put in 53-54 litres on both occasions.

Emissions conspiracy does not come into it...its just motor industry standard safety margin approach of showing 0 miles to empty when you still have between 3-4 litres left in the tank

To the OP comment about differing range estimates at end of tank, that is just down to the algorithm used in the calculation, I have driven with say 70 miles to empty along a 50mph average speed check area for several miles and at the end the miles to empty goes up to 75!!!, on the sams stretch of road with half full tank it shows something like 150 to empty at start and end of the 50mph section........ is just the maths not a conspiracy!!!!!

 

[kalniel]

Emissions conspiracy does not come into it...its just motor industry standard safety margin approach of showing 0 miles to empty when you still have between 3-4 litres left in the tank

To the OP comment about differing range estimates at end of tank, that is just down to the algorithm used in the calculation, I have driven with say 70 miles to empty along a 50mph average speed check area for several miles and at the end the miles to empty goes up to 75!!!, on the sams stretch of road with half full tank it shows something like 150 to empty at start and end of the 50mph section........ is just the maths not a conspiracy!!!!!

The emissions question was meant to be separate from the tank size/miles remaining question, sorry if I didn't make that clear. What happens is the indicated MPG average is nearly 10MPG higher when I'm in low fuel condition. Nothing to do with miles remaining etc. The question is why does my economy suddenly increase so much when in low-fuel state?

 

[Daveotto]

. The question is why does my economy suddenly increase so much when in low-fuel state?

If you are not driving around at 40mph then it is likely to be a fault so would take it in for Audi to have a look

 

[kalniel]

If you are not driving around at 40mph then it is likely to be a fault so would take it in for Audi to have a look

Indeed, this is dual carriageway so 60-70. Not sure what I'd say though "my fuel economy is fine, but gets even better when I'm low on fuel"? My garage can't even identify obvious faults so I don't hold much chance of them looking into it.

 

[airbus319]

Don't forget the unusable fuel you always have in the tank which can't be picked up by the pump inlet.

 

[glasgowdubber]

I drive a 2.0 TDI so I'm sure my tank is 45 litres, my light always comes on at 35miles, last night I brimmed it with 42.81l