C172 MISCALCULATES FUEL ON BOARD; DIVERTS TO A CLOSED ARPT; AND LANDS WITH A TOTAL OF 4 GALS OF FUEL.

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DID A PREFLT AND MADE A NOTE THAT I HAD 12 GALS IN 1 TANK AND 15 GALS IN THE OTHER. THE FLT IS APPROX 40 MINS. MY PLANE BURNS APPROX 7 GALS PER HR. THAT GIVES ME LESS THAN 4 HRS. I FLEW TO DEST AND BACK OVER MY FRIEND'S HOUSE (HE HAS WANTED ME TO TAKE A PICTURE). I NOTICED BOTH OF MY GAS GAUGES WENT TO EMPTY. I NEVER CONSIDER THEM TO BE ACCURATE; BUT WAS WORRIED THAT I HAD A LEAK OR A MISTAKE IN RECORDING MY FUEL. I CALLED A FRIEND IN ZZZ TO HAVE HIM PICK ME UP AT ARPT; FLEW OVER AND SAW THE X'S (CLOSED). I CALLED HIM BACK AS HE ARRIVED THERE AND ASKED HIM TO CHK THE FIELD. A MAN WAS FLYING 'RC'S' AND HE ASKED HIM TO HOLD UP SO I COULD LAND. I LANDED AND CHKED TANKS; 1 GAL IN R SIDE AND 3 GALS IN L SIDE. MY PLANE CAN BURN CAR GAS. I PUT IN 5 GALS AND FLEW TO ZZZ1 AS I HAVE AN ACCOUNT THERE. I PUT IN 30.5 GALS OF 100LL. WE FOUND NO LEAKS IN THE TANKS. IT APPEARS I HAD MADE A MISTAKE IN CHKING FUEL. I WILL BE MORE CAREFUL IN CHKING FUEL IN THE FUTURE. I AM GLAD I LANDED; I WOULD NOT HAVE MADE IT MUCH FARTHER.