A full tank (especially of gasoline). Informal expression for a tank filled to capacity.
出発前にガソリンを満タンにしておこう。
Let's fill the gas tank before we leave.
長距離ドライブなので、タンクは満タンだ。
For a long drive, the tank is full.
Full capacity — e.g., a battery or receptacle charged/filled to its maximum; also used figuratively for 'completely full'.
携帯のバッテリーが満タンになった。
My phone's battery is fully charged.
トランクは荷物で満タンだ。
The trunk is full of luggage.