Been using the same riding mower for 20-some years. I have no idea how many gallons it takes to mow my yard. I just know I can't mow it twice without refilling. But I sometimes do it anyway and run out...
I fill it when I start and go until it runs out, then refilled it.... The E3 spark plug seems to get better economy than the standard plugs...
You also gotta keep the air filter clean... Mine gets stuck with fuzzies and can make it stall.... So I blow them out when they get dirty to clean them up.... I've changed my air filter 3 times already this year... I have three filters that I rotate through...