The formula with 26 substituted
litres = 26 × 3.785411784 = 98.421
The constant is an exact definition, not a rounded factor: the mile is 1.609344 km, the US gallon is 3.785411784 litres and the imperial gallon is 4.54609 litres. The Gallons to Litres Converter explains where each comes from and gives a longer table.
26 US gallons is 98.421 litres. 26 imperial gallons, the gallon behind British mpg figures, is 118.198 litres.
What 26 US gallons means in practice
26 US gallons is 98.421 litres, the extended-range tank offered on several full-size pickups; 26 imperial gallons is 118.198 litres. At 20 mpg the pickup's tank is worth 520 miles between fills.