Fuel Guage and miles to go issue.

Happens all the time and also happened on my old Mondeo, however it also goes back up too.

So I wouldn't personally worry about losing a few miles (17), if it's losing more than a few miles then I'd start to worry.




Edited by Gazwats - 16 Jan 2012 at 7:30pm

Yes it seems to try to work out your range on recent mpg, it goes down while I do short trips and up on motorway journeys as you would expect. I wouldn't rely on it too much though as it seems to drop very quickly over the last quarter tank.

This just happened to me 2 days back!

Heh Niall, you bet I will really be considering moving over if things at work iron themselves out.

Going back to the original issue of what the 'Distance to Go' indicator displays . . . . .

Obviously, the indication can ONLY be based on two factors, namely current (perceived) 'MPG' rate and the known quantity of fuel in the tank. If you multiply these two figures together, you get 'miles to go'. Simples.

Except . . . .

The ECU must know, with pretty good accuracy, both of these values if it is going to provide a useful indication of DTG.

Fortunately, modern fuel tank sensors are pretty accurate - or they can be - and so at least one of the two variables can beknown with pretty good reliability.

Unfortunately this isn't the case for the MPG figure. It isn't so much that the value isn't known with any accuracy, or that it is difficult to obtain - this is far from the case. In fact, the two individual components that make up the MPG figure (distance and fuel flow rate) are, nowadays, very accurately obtained from on-board sensors that are critical to the engine management ECU.

No, the problem is what MPG figure is actually 'used' to calculate the DTG value.

Using some form of 'instantaneous' MPG figure would simply mean that the DTG value would be as erratic as the 'instant' (non-averaged, or non-smoothed) MPG indication. In other words, hammer up a hill in 3rd, and the DTG display might suggest that you won't even make it to the top, whereas coast down the other side in 6th, with the wind behind you, and the DTG might suggest that you needn't bother refuelling again this year!!

Similarly, if the calculation used an 'averaged' MPG that went all the way back to the day the car was bought, then that isn't much use either.

I believe that the DTG calculation is based on the 'averaged' MPG figure - which, in my mind, is the most sensible figure to use.

If you reset the averaged MPG value - for example at the filling station, having just filled up - then the 'averaged' MPG figure will itself be erratic for a few miles, until enough data has been acquired to start 'smoothing' it out. Until that happens then, naturally, the DTG figure will reflect the changes in the MPG figure.

This approach will work well because, at the 'end' of a tank of fuel, when DTG is perhaps needed most, it will be based on the most 'stable' data (i.e. well-averaged MOG information, and a well-calibrated fuel-level sensor in the tank).

Of course, if you are running your Kuga on rainwater, it does make the calculations that bit more difficult!!

Cheers,
Niall