Car MPG readouts need to be taken with a pinch of salt. Check fuel in against mileage driven over at least 1,000 miles, and compare.
On our last 2 cars, one was almost spot on, the other over read by 5.6%. Still checking on the latest car, looks to be about 4% overreading.
Which fortunately does not actually matter if you are merely comparing your own cars mpg figure between winter and summer,although I do always keep an eye on my actual fuel consumption fill to fill at the pumps !
