I have a Ford Fiesta Zetec 2012 and have noticed recently that the display on 'miles until empty' is going down very quickly when I the tank is fairly full, more so than when its less full.

I filled up with around 15 litres of petrol the other day which increased the miles from 150 to 340.

I then drove just under 8 miles the next day and the display had gone down to 290 by the end. Then I dove another 3 miles the next day and it went from 290 to 270.

I was driving as well as I could and basically keeping revs below 2000 the entire time, so is there something wrong or is my mpg just terrible? 

The trips were short, fairly stop and start journeys, but I cant imagine that would cause my mpg to become so bad since I was keeping revs low.

Any help?

To find out what is the issue, if any, you need to measure the amount of fuel you put in, and measure the mileage between fill-ups.  So do not rely on the car's electronics.

This is easy - fill the tank until the petrol pump cuts out, plus one more attempt to make sure it is actually full..... reset the mechanical odometer to zero, and drive as normal until you need to fill up again - repeat the above filling procedure, then note the amount of fuel used and the distance covered.

Only this, with a bit of maths, will give you an accurate MPG.

Once you have this, you can then investigate if the electronics are wrong or the fueling on the car is faulty in some way.

I've kept a log of every fill up on this car since I bought it with Litres/£ per litre/display mpg/Calculated mpg and mileage of course ( also temperature sometimes).    Predictably, the display mpg has mostly been very  opimistic compared with the actual figure with the occasional blip which I can't explain.  I try to always use the same filling station  and pump as cut off can vary between stations.  The car computer is only a guide I think.

Most notable is the difference between summer and winter (calculated) mpg as in my case with short journeys, the engine rarely reaches operating temperature and I get significant difference between the seasons e.g.  35mpg winter, 45+ mpg summer.

One other thing to bear in mind is how the dash mpg is calculated. It's basically a rolling average of the instantaneous readings it takes every x number of seconds during your journey.

If you are resetting the dash mpg just after you have filled up it will initally be very sensitive to small changes, it will become more representative after a longer period of time.

I used to keep a record when I had my 2019 ST-Line 140 and I used to get about 55-58mpg on the dash readout on my usual 35 mile commute (During the summer, about 4 mpg less in the winter)

I found the dash mpg was about 3mpg or so optimistic when compared to the brim to brim method.

I'm now driving a Mazda 3 which is very similar mpg wise and I usually have to wait for about 40miles after resetting the dash average mpg display before it becomes  representative of the real average mpg.