Hi Phil
Are you taking the figures from the on-board computer?
If so, I would question their accuracy.
A habit I picked up when employed and running company cars is to maintain a spreadsheet of fuel purchases including mileage at the time of the fill-up.
Even though now retired, I still do this and find there is ALWAYS a difference between the car computer figure and spreadsheet calculations. Sometimes this is quite significant.
OK, call me a saddo for still doing this but remember a decline in fuel consumption can be an indicator of engine or brake problems.
Hope this helps.
John