Are you therefore saying that you are getting 24 miles per US gallon?
New math?
Remember the imperial gallon is larger than the US gallon.
1 imperial gallon = 1.201 US gallons.
20 miles per Imperial gallon = 20 miles per 1.201 US gallons.
20/1.201 = x/1
x = 16.623 miles per US gallon.
Cliff