I did 375 miles on the last tankful with an average of 44.6mpg which was better than it has done.. I think I did vent slightly when I filled it as it's quite easy to accidentally hit the vent switch though I didn't purposely vent all the extra space.
This tank I didn't vent at all, but only got 39 litres in. So far it's averaging just over 40mpg (commuting) so it seems to have got better for the same type of driving conditions over the last 2-3 tanks. I wonder if that's because it's approaching 7000 miles now. I was only getting
280 miles to the tank before but the prediction so far on this tank is about 350 miles.