Unfortunately you got bamboozled.

50000 miles / 12 miles per gallon = 4166 gallons

50000 miles / 10 miles per gallon = 5000 gallons (833 gallons saved)

50000 miles / 60 miles per gallon = 833 gallons

50000 miles / 30 miles per gallon = 1666 gallons (833 gallons saved)