Good afternoon. This is a very trivial query.  I bought a 2013 1.6 TDI MK3 Ford Focus Zetec in June 24.. When I bought it the trip said 50.1 mpg. Since then it has steadily decreased to now it's 49 mpg. Now I know it's only an estimated mpg.  But would there be any reason as to why it keeps decreasing. Stupid I know. But I'm just curious. Thanks in advance

Engines run best at optimum temperature.  The warm-up time is longer in winter so the engine will run less efficiently for a longer part of each journey at this time of year.

You can reset that figure to get a more accurate short term figure.  I used to reset mine every time I filled the tank, for example.

In addition, the 'trip computer' is little more than a toy. The only accurate MPG figure you'll ever get is brimming the tank full of fuel, driving the car having reset your odometer to zero, then brimming your tank again and working out how much fuel it has taken to drive you the set amount of miles. I only ever 'brim' my tank, and can keep a very accurate record of MPG...

Since the winter is now here and we have dark mornings and dark evening I'd expect your headlights are on a lot of the time. Add to that the use of heated screens, heated mirror, heated seats, and the heater blower fan and I'd expect the MPG to drop at this time of year.

As above, it very much depends on the kind of driving you're doing. As an example I've noticed that the mpg tends to slowly increase on our Vitara when doing decently long journeys on secondary roads at speeds between 30mph and 60mph but drops back when cruising at dual-carriageway speeds and, of course, in urban conditions.

I haven't done any brim-to-brim tests for several years but the last time I did one I found the computer was fairly accurate, within 1-2mpg. Considering it can only measure the fuel dispensed by knowing the fuel rail pressure, injection timing and, presumably, the calibration of the individual injectors based on their coding I find that quite surprising. It probably also factors in the fuel density related to temperature too.

Irrespective of the accuracy it does, of course, give a good comparison of the consumption in different driving conditions as I said.