My worst is 65.30 mpg (54.37 mpg U.S., 4.326 L/100 km).
That was on February 1st this year and saw a lot of highway riding
My best is 89.85 mpg (74.82 mpg U.S., 3.133 L/100 km).
That was on July 22nd, 2018, which was with Sainsbury's 97 RON fuel, and was mostly single carriageway A-roads with a bit of highway riding at the end. It was also the time I forgot to do up my jacket pocket and lost the keyring for my top box and bike locks without realising until it was far too late.
I stopped calculating an average at the end of 2018 after Asda wrongly charged me for £10 of diesel (!), which saved me money but meant I have no idea how much I filled the tank with. It was one of those toll booth-style stations where you pay as you leave. So I have no idea what was said over the engine noise, and then I had to quickly get moving to not hold anyone up, so I never looked at the receipt until I got home.
Anyway, my tank-to-tank average to December 16, 2018 was 78.20 mpg (65.12 mpg U.S., 3.612 L/100 km).
It has definitely gone down a lot since then, probably because I now spend a lot more time on motorways, either to visit my dad since his illness or just getting to new places to ride.
I have also filled up my bike 191 times since bought it, from 190 different petrol stations.