My Volvo with a 13l uses 2.5 gal every 450/500 miles. On a 6 day work week have to fill up twice, one tank will not go the distance of running the full week. I have not worked it out against the fuel though, and the gauge seems to drop really quick after quarter of a tank. I always carry a back up box for emergency, but prefer not to get below quarter of a tank.
I’ll look at the fuel to def ratio this week.
I must admit I don’t pay to much attention as its not costing me a dime!