My point with litres is that all over the world a litre is a liter is a litre, all of them, wherever you are, are 100cl or 1000ml. (Plus, because we buy fuel priced by the litre here it's a consistent start.)
An imperial (UK, Canada, etc. - mainly ex-commonwealth) gallon is 8 imperial pints and is 4.54609 litres. It is also just about the same (temperature and atmospheric pressure dependant) as the now defunct US 'dry' gallon.
A US gallon is 8 US pints and is 3.78541 litres.
Over the many years, there have been nearly 20 different sized gallons in statute use - these ranged in size from roughly 3.5 to 4.7 litres.
Even comparing the two by using fluid ounces is off, there is a discrepancy between the two (US and imperial floz)...
1 imperial pint = 20 imperial oz = 19.2 US oz....
1 US pint = 16.65 imperial oz = 16 US oz...
It really doesn't seem a lot like that, but an imperial gallon = 160 imperial oz = 153.7 US oz. 7 oz different? That's nearly half a pint (in the US...) over a tankful you're getting a bit further out - give it 10 tanks for a longer term fuel efficiency calculation and you're well off.
The only thing is reliably consistent is the litre/liter, well, consistent except for the spelling of course.