When I purchased my Kia 1.4 D was brand new and was different. I will explain.
As I am driving mostly on Motorways and I am the only driver the car giving me different MPG.
Brand new to 30 000 miles I couldn't get more than 48MPG
Between 30 000 and 60 000 improved and I got 64 - 68 MPG
Between 60 000 and Present 97 000 miles I got 72 - 76 MPG.
The route is exactly the same for the last 5 years.
I have noticed that there is a difference in mileage when I use different petrol stations. The most effective fuel I use BP. I got more range compare then the rest petrol stations.