I have never checked my mileage the old manual style since I have been running diesels. I do know that based on the miles I travel when I stop and fill up my 6.0 F-250 pulling a Sunnybrook fiver averaged somewhere in the 10 to 11 range. My diesel pusher Cummins motor home ran in the 9.5 to 10.5 range. My 2017 6.7 F-350 is running in the 10 to 11 range as well.
In my opinion you should never buy diesel thinking it is going to be more economical than a gasser. I purchased my F-350 because I needed it for the pin weight, trailer weight and more than anything else for the stability of a dually. JMHO.