@daily.network.new: Fuel, like most liquids, expands when it gets warmer and contracts when it cools. This means cooler fuel is slightly denser, so the same measured volume can contain marginally more mass-and therefore slightly more potential energy-than warmer fuel. However, the idea that filling your car early in the morning will save you significant money is largely exaggerated, as petrol and diesel are typically stored in underground tanks where temperatures change relatively slowly throughout the day. The underlying science is real, but in everyday refuelling the difference is generally very small. It's an interesting example of how temperature affects materials even when we don't notice it happening.