Short Answer: Car A gets better millage
Car A
Distance = 300
Gallons = 8
Miles per gallon = miles / gallons
Miles per gallon = 300 / 8
Miles per gallon = 37.5 miles per gallon
Car B
Distance = 320 miles
gallons = 10
Miles per gallon = miles / gallons
Miles per gallon = 320 / 10
Miles per gallon = 32 miles / gallon
Car A = 37.5 mpg (all other factors equal) gives better millage. Bigger in this case is much better.