FORCED LNDG. OUT OF GAS.

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ON AUG/XX/95 I CONDUCTED A VFR FLT FROM THREE FORKS; MT; TO OGDEN; UT. A VFR FLT PLAN WAS FILED WITH BZN FSS VIA TELEPHONE. THE FLT WAS PLANNED FOR 3 HRS; 10 MINS AT CRUISE ALT OF 10500 FT MSL. THE TANKS WERE TOPPED VIA FUEL TRUCK (2) X 12.5 U.S. GALLONS (25 GALLONS USEABLE) EARLIER IN THE DAY; WHICH WOULD PROVIDE 4 HRS; 10 MINS BASED ON CONSUMPTION OF 6 GALLONS PER HR. THE FLT DEPARTED THREE FORKS AND PROCEEDED AS PLANNED. APCHIING TREMONTON; UT; CEDAR CITY RADIO WAS NOTIFIED AND THE FLT PLAN WAS EXTENDED AN ADDITIONAL 15 MINS; DUE TO UNFORECASTED HEADWINDS. DURING THE FLT; FUEL SELECTION WAS ALTERNATED AS USUAL BTWN L AND R TANKS FOR PROPER FUEL BAL. APCHING THE AREA NEAR WILLARD; UT; WHILE DSNDING THROUGH 8500 FT MSL; SLC APCH CTL WAS CONTACTED AND RADAR CONTACT WAS ESTABLISHED. SHORTLY AFTERWARD THE L TANK RAN DRY AND I SELECTED THE R TANK WHICH WAS EXPECTED TO CONTAIN MY RESERVE FUEL ABOARD. SINCE THE FLT HAD RUN 15 MINS LONGER THAN ANTICIPATED; I ELECTED TO PROCEED TO THE BMC ARPT WHICH WAS NOW APPROX 10 MI BEHIND ME. I ADVISED APCH I WAS REVERSING COURSE TO BMC AND I MADE A 180 DEG TURN TOWARD IT WHEN THE SECOND TANK APPEARED TO RUN DRY. I ADVISED SLC APCH OF OUR SIT AND TOLD THEM I WOULD BE LNDG ON I-15 JUST S OF BMC. WE LANDED ON I-15 3 HRS AND 26 MINS AFTER TKOF; WITHOUT DAMAGE TO THE ACFT OR PROPERTY NOR INJURIES TO MYSELF OR MY WIFE. THE ACFT WAS FUELED; INSPECTED; AND WAS FLOWN TO ITS HOME BASE (OGD) THE NEXT MORNING. I'M THE OWNER OF THE ACFT AND I'VE DETERMINED THE CONSUMPTION OF 6 GALLONS OF FUEL USED PER HR IS A VERY REALISTIC NUMBER. THE FUEL CONSUMPTION FIGURES PUBLISHED IN THE 1967 CESSNA 150 PLT MANUAL (CONTINENTAL 0-200-A ENG) ARE SUBSTANTIALLY LESS THAN THE 6 GALLON FIGURE I USE. THE TOTAL ELAPSED TIME OF THE FLT (VERIFIED BY GREAT FALLS FSS AND UTAH HWY PATROL) WAS 3 HRS; 26 MINS. I'VE BEEN PRUDENT IN MY FLT PLANNING AND PREPARATION THROUGH THE PAST 30 YRS AND 20000 HRS OF ACCIDENT/INCIDENT FREE FLYING. I'M LOOKING AT ALL POSSIBILITIES WHY THE ACFT WAS ONLY ALOFT FOR 3 HRS; 26 MINS INSTEAD OF THE EXPECTED 4 HRS; 10 MINS. THE ACFT SAT ON THE RAMP ALL AFTERNOON WITH THE RAMP TEMPS NEARING OR EXCEEDING 100 DEGS F. FUEL VENTING IS ALSO A POSSIBILITY.