Change in the weather.
 

We all know that fuel consumption is poorer in the winter than the summer, but after enjoying several weeks of warm, dry, early summer weather, this past week has seen temperatures here down by 10°C, with rain the past two days. What I have found is that the car is not getting anything like the mpg it was getting (going by ScanGauge E) when the temperatures were higher. My coasting points are all needing re-calibrated, as the car doesn't coast as freely (either in or out of gear) as it did when the weather was warmer. The joys of a Scottish summer!

It became summer a few weeks ago and will be nearly 90, and mostly well into the 90's, for the next almost 4 months. We get about 6 months of that and the other 6 months encompass the other 3 seasons.

What they say in Scotland is, "If you don't like the weather, wait ten minutes". We can get all four seasons in one day. I have photographs of heavy snow, at sea level, in June! Away from the coast we can get frost, hard enough to kill off plants, in July. The storm we had on Thursday blew down two mature trees near me! 65 mph winds on the Queensferry Crossing.

I concur about poorer fuel economy in cold weather. The "winter gas" myth always gets the blame, but the actual culprit is that everything stiffens up, and created lots more resistance. People often limit their thinking to engine oil, but there's also transfer cases, differentials, universal joints, transmission, wheel bearing, belts and pulleys, etc. And then, if the weather is really cold, add fans, a rear window defroster and heated seats, plus possibly headlights for shorter days, all of which take a toll on the batter, which requires fuel to recharge.

I live in southern Ontario, Canada, and I've done 1,100+ mile road trips to sunny Florida. I've left the house with the temp around 40'F (~4.5'C), and spent the night in the Virginia mountains, having a cold-soaked engine in freezing weather. By the end of day 2, in sunny Georgia with temps around 75'F (24'C) real-time fuel economy was substantially higher.

The arguments continue. I've done that run often enough to see the difference that ambient air temperature can make.

What surprised me was that with an air temp change of probably less than 10°C (no more than 22°C in the sunshine to 12-14°C now) made such a noticeable difference. And we are talking about an overnight change. You wouldn't think the countryside and roads would lose temperature so quickly. The car is garaged when it is not being driven, so it probably experiences an even smaller change in temperature. But still enough for the ScanGauge E (and my coasting experiences) to register a difference.
I think the biggest difference must be in the temperature of the Tarmac (Asphalt) on the road surface, and the effect it has on the tyres. It effects F1 racing tyres, so must do the same for road tyres.

Still managed a Fuelly calculated figure of 63.8 UK mpg (53.1 US mpg) for the tankful. My ScanGauge E was slightly pessimistic with it's estimated figure being 61.4.