I've owned my car almost 6 weeks and covered over 6400 kilometres or close to 4000 miles. Until yesterday my driving had been very focused on economy, the temp has been in and around the 70's and 70% of my trips highway at the 62 mph limit.
Friday I did a 550 kilometre (340mile) return trip. Returning home on Friday afternoon I wanted to get across the city before rush hour so I disregarded the economy and cruised at around 120-30 K/ph or 75mph. With 920 Kilometres or 570 miles on the tank I filled up (85% highway). (Right or wrong when I fill up the gas is sloshing in the neck, it's the only way I know to ensure a little consistency.) 50.37 litres or 13.31 US gallons was all I could get in the tank. According to this
calculator that's 5.47 litres per 100 kilometres or 42.99 miles per US gallon. This by far my best economy.

Was my average before the fast run home that good or did the higher cruising speed not have that bigger impact?
BTW I only took note of wind on Fridays run and basically it was from the side in both directions.

 

Interestingly, I just read a news article about fuel pump accuracy at gas stations in Washington State. A few inspectors go around and test each pump for accuracy, filling a 5 gallon graduated bucket twice: once at a slow rate, once at the fastest delivery. I don't recall the acceptable range, but it was something like +/- 5%. The inspectors report most pumps actually deliver more fuel than reported.

Could it be that you had a pump that showed a lower delivered amount than you actually got? Just a thought.

 

Differences between gas quality (more methanol=less energy), pump errors, elevation, traffic, wind, all affect fuel economy and it doesn't take much to go from let say 38mpg to 42, so I would say you're within margin of error and driving slower doesn't mean you get better fuel economy. It is very often that I find myself getting better fuel economy when I don't care , than when I actually try to get it better. My older Honda Accord was always getting best mpg in mountains, totally opposite to what I would expect. From my experience I think really heavy penalty for driving very fast doesn't show up until you start getting to 80mph and above and I usually get best mpg between 60-70 mph, unless I can keep it around 40 and use full electric part of the way. It's nice to have fuel economy better than subcompact in midsize sedan, isn't it?

 

I was telling someone the same thing the other day. It's contrary to what one would think, but if I change the dispaly screen or just don't look at it for a while, I really surprise myself when I see that my fuel economy has gone UP! Unfortunately, I have a hard time ignoring it for any significant length of time.