Using three different units for fuel calculations (litres, Imperial gallons and US gallons) is fraught with risk. Compound this with the useless fuel gauges in most light aircraft and pilots' reluctance to pay for more fuel than is absolutely essential and I'm surprised that there aren't more accidents due to fuel calculation / uplift errors.
When marking Flight Performance and Planning papers for the PPL, I was surprised how many people didn't understand the difference between density and specific gravity, particularly when using US gallons.
We had an aeroplane with a generally accepted 4 hour endurance, but we had an absolute rule concerning minimum in flight fuel indications. A pilot flew it for an hour on an instructional GH flight, then a renter took it on a 90 trip to Wales. He didn't bother refuelling whilst there and despite reaching the minimum indications, pressed on back to base passing a number of available aerdromes en route. At 1+(2 x 1½) = 4 hours, some miles short, it all went quiet and he had to land it in a field....
Fortunately without damage.