The imperial gallon is the gallon of the British imperial system. NIST's conversion factors list the "gallon, Canadian and U.K. (Imperial)" as exactly 4.54609 liters, or 4.54609 × 10⁻³ cubic meters. It is larger than the US gallon of 3.785412 L.
Conversion
L = imperial gal × 4.54609
imperial gal = L ÷ 4.54609
US gal = imperial gal × 1.2009
Example
A 10 imperial gallon quantity is 45.46 L, while 10 US gallons are 37.85 L, a difference of 7.6 L. A 50 L tank is 50 ÷ 4.54609 = 11.0 imperial gallons but 50 ÷ 3.785412 = 13.2 US gallons.
Imperial fluid ounces
An imperial fluid ounce is 28.41306 mL (NIST), smaller than the US 29.57353 mL, and the imperial gallon holds 160 of them against 128 in the US gallon.
See how many liters in a gallon for charts and metric vs imperial for other near-twin units.