AN INSTRUCTOR ALONG WITH 2 OTHER PLTS; AFTER A XCOUNTRY FLT; LANDED WITH ONLY 2 PT 5 GALLONS OF FUEL REMAINING.

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C172; 180 HP; DEPARTED BKT; VA; APPROX XA00 WITH 3 PLTS ON BOARD -- 2 WITH OVER 1000 HRS; THE PF (L SEAT) HAS APPROX 500 HRS. R SEATER IS INSTRUCTOR (YOUR RPTR). BACK SEATER CHKED FUEL; RPTED FULL (BOTH TANKS COMBINED HAVE 40 GALLONS USABLE). IN PREFLT PLANNING; WE KNEW WE HAD 40 KT HEADWIND; ESTIMATED 3.5 HRS FLYING TIME; WOULD USE 10 GALLONS FUEL PER HR; WITH 30 MINS FUEL REMAINING. R SEATER SUGGESTED A REST/FUEL STOP AT SOP; NC. PF SAID HE WANTED TO GET BACK QUICKLY AND DID NOT SEE ANY NEED TO STOP AT SOP; NC. WIND AT CAE (DEST) OUT OF SW GUSTING TO 24 KTS. PF CRUISED AT 2600 RPM AT 6000 FT MSL. A CALL CAME FROM A COMPANY PLANE SAYING THEY HAD LANDED IN FLO FOR FUEL; TAKING ON 31 GALLONS. WE ACKNOWLEDGED THE CALL AND CONTINUED ON TOWARD THE DEST. THE FUEL GAUGES READ; L TANK 1/2 FULL; R TANK NEARING EMPTY. AT CAE WE WERE CLRED FOR THE APCH TO RWY 5 VISUAL. OUR L TANK WAS STILL 1/2 FULL (SHOWING) WITH R TANK APPEARING TO BE ON EMPTY. WE LANDED AT CAE. AT REFUELING; WE TOOK ON 37.5 GALLONS; MEANING WE ONLY HAD 2.50 GALLONS OF FUEL REMAINING. IF THE ACFT WERE FULL (40 GALLONS; USABLE); WE BURNED 11.36 GPH. ENDURANCE CHARTS SHOW A BURN OF 10.1 GPH AT 6000 FT AT 2600 RPM. I DON'T THINK ANY OF THE 3 OF US WERE SURPRISED. THE PF SAID 'THERE'S GOT TO BE SOMETHING WRONG WITH THIS AIRPLANE.' I TOLD HIM HE SHOULDN'T TRUST THE ENDURANCE CHARTS. ON REFUELING; WE FOUND THAT WE HAD 1.25 GALLONS PER TANK -- EVEN THOUGH THE L TANK STILL SHOWED 1/2 FULL WHEN WE STOPPED THE ENG ON THE RAMP. I FOUND THAT CONTRARY TO MY OWN TEACHING; I WAS HOPING THAT THE L GAUGE WAS ACCURATELY READING FUEL REMAINING. BUT I HAD A TERRIBLE FEELING THAT THE GAUGE WAS WRONG; BUT I DIDN'T KNOW HOW WRONG.