Avgas is about 0.1% per degC in volume so if you fill right up with fuel which ITSELF is say -10C then you are carrying 3% more energy than if the fuel was say +20C.
In reality the only way to achieve that is to fill up, leave the plane outdoors at -10C and then when the fuel has shrunk you top off the tanks
Then go flying right away before the tanks warm up and the juice leaks out of the vents