We need to clarify the GPH with Heather Lawn SP.
If the burn rate is 40% less, or a gallon less per hour on a 72" mower, then you save about $4/hour. That's the price of a gallon a gas plus the lower cost of a gallon of ag diesel. If you pay more for diesel, figure you'd save about $3.5/hour.
So $1200 to 1400/year in gas. Probably a bit less on a 52/54" mower, but still significant.
I'm not clear on the numbers from yet.