Fuel/Trip Computer Inaccuracies...

[Space Coyote]

I've had my Leon TDi Sport since May 2006 and while I'm very happy with it overall, I do have a gripe about the fuel/trip computer; more specifically the range function and how inaccurate it is and how quickly it drops, especially when the tank approaches empty.

Does anyone know how the range is calculated? My thoughts are that it should take the average MPG and then apply some slight modifier according to instant/trip average MPG and make a conversative interpretation of the range.

What seems to happen though is when I fill up the range seems to suggest I have between 580 and 600 miles range and this optimistic view is applied pretty much all the way down to the 1/8 tank marker when suddenly it will drop from 90 miles to 50 miles (with light) down to zero in the space of about 22 miles.

I appreciate that it must be hard for a sensor to give an accurate view of how much fuel is in the tank because it is a liquid and constantly moving. When the tank is full though, that should be easy to measure and the computer must have a good indication of how much fuel is being delivered to the engine (and returned to the tank - Does this actually happen?) and give a more accurate indication of the range?

Any one else see this inaccuracy?

 

[seremotors]

yep - on every car I have ever owned the gauge always took ages to go from full to half then it dropped from half to empty like I had a hole in the tank!!! I assumed it was to do with the shape of the tank and the angle of the float making accuracy very difficult?
On my Leon, I ran low on fuel and the range seemed to drop by about 15 miles every 5 . . . . . .

 

[andycupra]

the predicted miles left is usually based on the very recent mileage achieved. say 10 miles or so, and so this can lead to optimistic predictions. - this is partly as people tend to look at the miles left at the end of journies and not the beginning when you are getting less mpg. so it will say 100 miles to go based on the last 10 miles driving. But if you are doing 15 miles journies you will not achieve this as the car will be running from cold every 15 miles.

So if you parked up after a reasonable drive and though, great another 90 miles to go, then got in the car in the morning done a few miles and though wow its not 50 this is due to the cold running.

So, the fact its not based on say 500 miles of driving means that the prediction can change and vary alot over a short space of time.

The other thing that doesnt help if that half on the gauge is not typically half. You can get the guage to 'full' and then get another 10+ litres in, so early calculations may be optimistic.

 

[Space Coyote]

Yeah, I see what you're saying and the algorithm the computer uses has to be a compromise because the car doesn't know what the next journey is going to be.

Even so, I suspect that the majority of people's day-to-day journeys are very similar and this could be factored in.

The ECU "learns" your driving style, why doesn't the trip computer?

EDIT: Maybe I'm asking too much from a simple calculator huh?

 

[idge]

i find because it thinks im doing about 6-7 mpg more than i actually am the predicted range is very high.. Then after half way it starts to realise that its way off and 250 miles remaining is more like 150.

I've done a 2 hour journey where the last 30 disappeared in 5 miles!

 

[OLDOILER]

You should try and compare the speedo, mines reading 5 mph slow across the speed range! - this will affect ALL other calculations too !!

 

[andycupra]

You should try and compare the speedo, mines reading 5 mph slow across the speed range! - this will affect ALL other calculations too !!

your car speedo reads too low? you sure?
have you changed your wheels / tyres?

most seat cars read 2mpg fast at 30 up to 5 mph fast at 70...

 

[Space Coyote]

I am pretty sure that my speedo is reading about 10% faster than my actual (GPS) speed from zero.

As an aside, in the car I rented in the US last year, the speedo showed actual speed. Why is it that the UK have this feature, and is it restricted to the UK?

Also does this mean that I have 10% more miles on my car than it has actually covered?

 

[andycupra]

I am pretty sure that my speedo is reading about 10% faster than my actual (GPS) speed from zero.

As an aside, in the car I rented in the US last year, the speedo showed actual speed. Why is it that the UK have this feature, and is it restricted to the UK?

Also does this mean that I have 10% more miles on my car than it has actually covered?

easy way to check, on next long journey set the cruise to 60mpg exactly on the speedo, then for every minute the miles covered should go up by 1 if the speedo was acurate.
If after 5 mins they still match then the mileage is wrong and increasing at the rate the speedo indicates.
however if after 5 mins the mileage has only gone up by 5.5-5.6 miles then this would indicate that the miles covered is more accurate than the speedo.

ps the dash actually calculates in metric, (KM) and does a conversion.

 

[OLDOILER]

More than likely!!

 

[Space Coyote]

ps the dash actually calculates in metric, (KM) and does a conversion.

I didn't even consider that there may be some conversion in there somewhere.

 

[stevesulsh]

speed in kmh on mine checked against my sat nav which i assume has to be accurate there is a + 10% error. checking my fuel consumption based on fuel used to fuel light on from last fill and kilometres travelled on sat nav getting an ave 8.9 litres/100 km is there any way of getting speedo etc recalibrated??