How does the 'Distance until empty' thingummy work?

[zcacogp]

Chaps, 

Left-field question here. How does the "Distance until empty" display in the dashboard work? Obviously it tells you how far it thinks you can go on the current tank of fuel but how does it calculate this? It will clearly use an economy figure based upon a number of factors but what are those factors? 

The reason I ask are that it seems to jump around quite a lot. It seems to gently creep up after filling up with petrol and stabilises when the tank is 1/3 to 1/2 empty. However I saw it lose 18 miles in less than 50 yards the other day, just after startup. (This drop was so steep that I turned the car off and checked for a fuel leak. Yes, really.) That was with a 3/4 full tank as well. 

I also did a fairly brisk journey of 100 miles a couple of days ago which cost me over 160 miles in 'distance to empty'. Which seemed a little depressing. 

I can understand that it will probably use a long-term average MPG but how long-term is this? It's NOT the MPG displayed on the dashboard as this is very stable for me, but the 'distance to empty' moves around a lot. 

And what other factors does it use? Temperature? Altitude? Fuel type (from engine knock sensors)? And does resetting the trip reading on the dashboard reset these parameters? 

All thoughts appreciated .... thanks! 

Edited July 5, 2022 by zcacogp

[brillomaster]

i didnt think it used long term figures at all, rather it uses pretty much instantaneous figures. so immediately after start up when you're getting an mpg in the high teens, the DTE will reflect this. then when you've got the engine warm and your MPG improves, it'll jump up. how much it jumps up by will obviously be dictated by how many gallons of fuel its multiplying the mpg by.

i often find my (330d) cars range with a full tank will be 480 ish miles, but do 20 miles of 50mph average speed, and the DTE will rise to over 600. but once normal 70mph cruising is resumed it'll drop back down to 450 or so.

 

[zcacogp]

Thanks brillomaster. 

I think that it must use longer-term MPG figures. If it used instantaneous MPG figures then it would drop hugely when starting on a cold day, when the car uses a LOT of petrol to get started and warm up, and then increase massively once it is warm. It does increase when warm but not that much. 

If it used instantaneous figures then it would also show single-digit DTE when you accelerate hard but an almost infinite figure when coasting down a hill; two scenarios which use vast amounts of petrol and no petrol at all respectively. (This can be seen clearly if you drive a car with an 'instantaneous MPG' read-out on the dashboard). In practice it does neither. 

I can understand that it would drop the DTE by a few miles when you do a cold start, as that start uses a quantity of petrol. However the drop of the DTE should correspond fairly closely to miles driven after that if it gets it's maths right. Generally this seems to be the case, but I wondered how it did the calculation. 

[Menoporsche]

I think it uses a rolling average of the last few minutes’ fuel usage, rather than instantaneous. Had a time where I had to catch up with the wife, I think from a toll where she had a tag and I had to stop and pay. I caught her in ? 5 miles, but lost 20 miles in range….

[Ninesomething]

I've given up looking at it. Too depressing. Putin's got to go....

[brillomaster]

Yeah dont look at distance to empty. Far more important to know the outside temperature in my opinion, gives a good indicator of how grippy your tyres will be.

[moospeed]

This was 40 miles after filling up and straight onto the motorway.  I’d gained about 100 miles of range gradually during that 40 miles.

Each time I took a picture and didn’t think it would go higher but it did.

 

[entdgc]

On 7/5/2022 at 1:40 PM, Menoporsche said:

I think it uses a rolling average of the last few minutes’ fuel usage, rather than instantaneous. Had a time where I had to catch up with the wife, I think from a toll where she had a tag and I had to stop and pay. I caught her in ? 5 miles, but lost 20 miles in range….

This ^^^