I fill the tank regardless of cost as I use it fairly rapidly even at 65+mpg. I can more easily aim for 600-650 miles per tank full.
I have a new target though, I read on EcoModder that there is a chap with the same engine as mine getting 94mpg. My best was 84mpg (on extra virgin olive oil).
I did try only filling half a tank but it meant a lot more trips out of the way to fill up. The extra weight doesn't make any difference to economy, with my car, but would with handling.
I monitored it for 5 years with part tank, full tank, empty boot, boot full of tools and many other variables and there was more variation due to weather then my tank of fuel or weight of tools.
Windows open increased consumption by 10% where as roof bars added 15% even with no load on top. Air con or not made no difference as it is climate control and it continues to run the pump as required.