What's your route and what indicated speed are you riding at wide?
In my old job I was doing 9 miles each way of dual carriageway with plenty of roundabouts and was sitting at indicated 80/5k rpm which returned 46-49mpg. Changed jobs and now do 25 miles each way including M4 and sit at indicated 76/4.5k rpm which returns 57-59mpg.
Before, I was like you and would be filling up at 165/180 miles, now it's generally 215 before it starts flashing.