Last week I filled up with petrol in London and drove up to my home near Newmarket. A journey of about 70 miles - about halve on the M11 and most of the rest on good quality A roads, with the last 5 miles or so on country lanes. After filling up, I zeroed the mpg meter and was rather amazed to see that I averaged an indicated 35.6 mpg for the trip. Then I realised that the climate control was switched off so no a/c on.

Today I did exactly the same journey, filled up at the same garage, and drove up under very similar conditions - except that I had the climate control (and a/c) on and the ambient temperature was about 5 deg C cooler. This time I averaged 32.0 mpg. Still not bad, but 10% worse than last week.

Interesting.
On warm days I like to switch the climate control off and have the windows and sunroof open but apparently that's even worse for fuel economy because of the increased drag.....

Not sure how accurate that may actually be - until you do a proper 'brim-to-brim' test.

I know the OBC is fairly reliable, but it can mis-estimate sometimes