Fuel Gauge/Range Accuracy

[loj94jdn]

Morning All (2016 81TSI)
So I have been questioning a while the accuracy of the fuel gauge and range remaining counter.

I drove it to 5km range remaining yesterday and then stopped to put $5 worth of fuel in (3.2litres) - which I add isnt my norm but its expensive day and I usually fill up on Mondays!

By my calculations that about 60-70km of range based on what I usually burn. However the 3.2 litres wasnt enough to register I had put anything in so the range stayed at 5km and very quickly dropped to zero and the needle didnt move.

So 2 questions -1. have I screwed the calibration on the gauge and 2. whats the accuracy of the range remaining and fuel gauge - has someone driven to empty and worked out the leeway built in?? Suppose this could go on and theres no real benefit in knowing to be honest rather just curiosity!

[Sydneykid]

Like most cars VW fuel gauges are conservative, they show pessimistic fuel levels and understated range to empty. Polo's have rather flat and wide fuel tanks so the fuel moves around under cornering, braking and acceleration which changes the fuel gauge reading quite a bit. Fuel gauges aren't instantaneous in their response, they have a lot of dampening so that the sloshing around doesn't make the gauge move. As a result of all of those factors there is quite a bit of "safety margin" built in, VW don't want us complaining that the car stopped, out of fuel, when the gauge said 10 k's to go. I have driven 20, 30 even 40 k's (on the freeway) with the range showing zero, so my guess is there's around 3 to 5 litres of "safety margin" built in.

To answer your questions, adding 3 litres is probably within the "slosh" range so the gauge simply ignored it.

Cheers
Gary

[amazeer]

I've had mine on 0kms a few times. It gets pessimistic overnight. I can park it with 10kms showing and then say 0 the next day.

I've driven on 0kms for 10kms and put 42 litres in. One of those freeway moments when you think you'll make it to the next stop but then pucker up when it becomes clear you might not.

My speedo is correct because I have oversize tyres. If you have the stock tyre size with the 7% inaccuracy I'm not sure if that would make it more pessimistic or optimistic. Either way I wouldnt recommend chancing your luck.

[Sydneykid]

I've had mine on 0kms a few times. It gets pessimistic overnight. I can park it with 10kms showing and then say 0 the next day.
I've driven on 0kms for 10kms and put 42 litres in. One of those freeway moments when you think you'll make it to the next stop but then pucker up when it becomes clear you might not.
My speedo is correct because I have oversize tyres. If you have the stock tyre size with the 7% inaccuracy I'm not sure if that would make it more pessimistic or optimistic. Either way I wouldnt recommend chancing your luck.

As I do with every new car, I've compared the digital speed read out with the actual (time over distance) speed and the difference is ~3% optimistic with new tyres. As the tyres wear (get smaller in diameter) that optimism will increase of course. There is a slight difference in what the speedo says the speed is and what the odometer says the distance travelled is, but it's very small.

The movements from 10k's to go to 0k's is most likely the fuel in the tank settling overnight (no slosh) while stationary, there may also be a bit of contraction due to the lower overnight temperature, especially if it's outdoors.

In regards to when the car is going to run out of fuel (safety margins aside), it's neither optimistic or pessimistic, it will travel the number of k's indicated by the display. If the speedo/odometer are reading low (compared to actual) then it could be considered optimistic when compared to the actual distance to the destination. (eg; sat nav says 100 k's to go and fuel range says 103 k's).

Cheers
Gary

[amazeer]

As I do with every new car, I've compared the digital speed read out with the actual (time over distance) speed and the difference is ~3% optimistic with new tyres. As the tyres wear (get smaller in diameter) that optimism will increase of course. There is a slight difference in what the speedo says the speed is and what the odometer says the distance travelled is, but it's very small.

The movements from 10k's to go to 0k's is most likely the fuel in the tank settling overnight (no slosh) while stationary, there may also be a bit of contraction due to the lower overnight temperature, especially if it's outdoors.

In regards to when the car is going to run out of fuel (safety margins aside), it's neither optimistic or pessimistic, it will travel the number of k's indicated by the display. If the speedo/odometer are reading low (compared to actual) then it could be considered optimistic when compared to the actual distance to the destination. (eg; sat nav says 100 k's to go and fuel range says 103 k's).

Cheers
Gary

From new on the original contis the speedo read 7% over GPS speed and I think its pretty typical. It **** me, so I went up to 45 series which made it pretty much accurate. The change with tyre wear is very minimal.

This is probably in the manual but at what distance or time is the range calculated? Range must be calculated based on your average consumption over previous 20km or 50km or 20 minutes, something like that. Under the right conditions I can get the range + distance travelled up around 800 but in reality for me anything over 650 per tank is a bit special.

[Sydneykid]

From new on the original contis the speedo read 7% over GPS speed and I think its pretty typical. It **** me, so I went up to 45 series which made it pretty much accurate. The change with tyre wear is very minimal.

This is probably in the manual but at what distance or time is the range calculated? Range must be calculated based on your average consumption over previous 20km or 50km or 20 minutes, something like that. Under the right conditions I can get the range + distance travelled up around 800 but in reality for me anything over 650 per tank is a bit special.

The maths is actually pretty simple. A brand new 215/40/17 is ~604 mm in diameter and an equally new 215/45/17 is ~625 mm in diameter, that's around a 3.5% difference.

A new car tyre has ~8mm of tread and the minimum legal tread depth is ~1.5mm. So if a new 215/40/17 is 604 mm in diameter then a just illegal one is 604 - 6.5 - 6.5 = 591 mm. That’s a 2.2% reduction in diameter.

So the difference between a worn out 215/40/17 and a brand new 215/45/17 is 3.5% + 2.2% = 5.7%.

I'm looking at replacing the 215/40/17's when they wear out with 215/45/17 mostly for their greater availability. Michelin's are my tyre brand of choice and they don't make a 215/40/17 that I have found, so going to the 215/45/17 will solve that issue. Plus give another 10 mm of sidewall which will help with the ride comfort a little.

From a brief chat with a tech at VW, the logic of the fuel range is that it is constantly being recalculated, whilst the display is updated when that reveals a significant difference (in the range).

Cheers
Gary

[weonarok]

The range takes its information from the fuel gauge.

The fuel gauge sender in the tank is basically a fairly crude (but surprisingly effective) wire arm with a float on the end. From one vehicle to the next a change of a couple of mill at the end of the arm will give a significantly different reading at the gauge for the same amount of fuel, therefore a 'true' range will also be effected.

My cook runs our Gen 4 Golf TDI down to 0 range quite often to my frustration as it would be a pain (and potentially dangerous) to run a diesel out of fuel on the road, but luckily hasn't happened in ten years. [emoji12]

Often she put 53 liters in it and once over 56, mind you, specs say 55 liter capacity. [emoji15]

So, as I said, every vehicle can be different and you need to familiarize yourself with your particular one by experimenting over many tank fills.


"You don't know what you don't know"

Cheers dave

[Sydneykid]

Often she put 53 liters in it and once over 56, mind you, specs say 55 liter capacity.

The quoted fuel tank capacity is the capacity of the tank itself, which leaves out how much is in the filler tube. With a fuel tank under the rear passengers seat the filler can easily hold 2 or 3 litres above the tank capacity. So she was pretty close to running out. My boss is the reverse, she panics when it gets to 100 k's to go

Cheers
Gary

[weonarok]

The quoted fuel tank capacity is the capacity of the tank itself, which leaves out how much is in the filler tube. With a fuel tank under the rear passengers seat the filler can easily hold 2 or 3 litres above the tank capacity. So she was pretty close to running out. My boss is the reverse, she panics when it gets to 100 k's to go

Cheers
Gary

Hahaha. [emoji104]

The Golf hatch tank is in the boot with the well for the 'flash' space saver spare, so the neck small and short. [emoji87]

Yep, but I can't say she was running on fumes because diesels would object to that. [emoji848][emoji85][emoji23]




"You don't know what you don't know"

Cheers dave

[Sydneykid]

Hi Dave, from memory (it's been a while since I worked on the Production Car Mk4 R32) the in tank fuel pump access is under the rear seat and the filler tube is around 4.5 cm ID and a bit more than 120 cm in length. That's around 2 litres in the metal tube itself, plus the rubber joiner and the twin breather tubes, so I'll stick by my 2 to 3 litres.

Or is the TDI fuel tank different?


Cheers
Gary

[weonarok]

Hi Dave, from memory (it's been a while since I worked on the Production Car Mk4 R32) the in tank fuel pump access is under the rear seat and the filler tube is around 4.5 cm ID and a bit more than 120 cm in length. That's around 2 litres in the metal tube itself, plus the rubber joiner and the twin breather tubes, so I'll stick by my 2 to 3 litres.

Or is the TDI fuel tank different?


Cheers
Gary

Doh!!! Without looking you may well be right. [emoji85] I was thinking of another car I owned in the past. [emoji30]

Two things come to mind: one I know little about my Golf, and two: the reason I know little about my Golf is that nothing has ever really gone wrong with it. I replaced the timing belt last year only to find for the first time that it is in fact a DOHC engine, and a couple of months ago I replaced all four half-shaft boots. That's about it in over ten years. [emoji41]

Now roks I do know little more about, from completely stripping two down and rebuilding one. [emoji87][emoji848] Why you ask? Well, because I could. [emoji104] Updated from MY11 to MY14 with far more bells and whistles and from manual to auto. [emoji41] So, does anyone have a spare V6 TDI laying around? Hahaha.

Ok. Back on topic. [emoji722]


"You don't know what you don't know"

Cheers dave

[mrkris]

they arent very accurate at all. ive driven into some servos with 20km estimated and the amount to fill up has varied from 40-44 litres

you never know what will happen so i tend to fill up before i get to the red zone.

[Mountainman]

My late 2015 81TSI manual Polo appears to have an accurate fuel guage taking approx 23L at half full of what is stated a 45L tank. The lowest range I have filled at a few times has been 25km with the guage indicating half way through the red where it takes 41L. I would hate to try to see what would be required to fit 45L in it.

Our Golf Mk7 on the other hand easily takes more than its official 50L, once taking 52.76L at the pump cut off with 5km showing on the range.

[Sydneykid]

Our Golf Mk7 on the other hand easily takes more than its official 50L, once taking 52.76L at the pump cut off with 5km showing on the range.

Sounds about right, a 50 litre tank plus 2 to 3 litres in the filler less 1/3rd of litre to do 5 k's

Cheers
Gary

[Guest001]

I've had mine on 0kms a few times. It gets pessimistic overnight. I can park it with 10kms showing and then say 0 the next day.

I've driven on 0kms for 10kms and put 42 litres in. One of those freeway moments when you think you'll make it to the next stop but then pucker up when it becomes clear you might not.

My speedo is correct because I have oversize tyres. If you have the stock tyre size with the 7% inaccuracy I'm not sure if that would make it more pessimistic or optimistic. Either way I wouldnt recommend chancing your luck.

What size difference is there in your tyres because when I did the comparison at 7% they would need to be 45mm larger which in Qld is illegal