V40 mpg dropping off a cliff

[denecaster]

I have a 64 V40 D4.

When I got it in May this year, the average mpg on the dashboard was something like 49. Over the summer, that gradually rose to a peak of 54.9, but settled at 54.6, doing a mix of mostly motorway driving, but some country roads and the occasional short local trip.

The actual mpg, calculated using mileage and how much fuel it took to fill a tank, was around 54-56.

During October, I was off work for a month with an injured shoulder, and most of my trips were local.

Obviously, the mpg fell.

I've been back at work for a month now, and it's back to a mix of motorway and country roads

But the mpg has continued sinking like a stone. The dashboard is now saying 51.6, and after just filling up, the most recent mpg was 46!!

Eh??

I must be doing something wrong somewhere. That's nuts.