760 GLE 1987 gas consumption

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hi , i just bought this 760 GLE and i know its V6 but it cost me wayy to much money for gas. i drive 80% of the time on highways and i pay the same amount than my friend driving a chevrelet express 2500 V8 .
There MUST be a problem cause i know im not fool and my friends thinks it
drinks too much too.