Actually it is safer to keep your tank full to avoid any explosion. It will reduce the amount of air in the tank and thus it is limiting the oxygen supply.
National Geographic Channel did an actual experiment using four different cars. The first one was full of gasoline and the last one was nearly empty. When they tried to explode the cars, the one filled full of gasoline didn't even explode, while the empty one made the biggest explosion.
The vapour inside the fuel tank was responsible for the explosion. Liquid gasoline in high temperature vapourises. 15:1 is the right ratio for an explosion. 15 parts of air and 1 part of vapour. So the higher the amount of gasoline the lesser the amount of air. So fill your tank to the full capacity during summer.