A C172 pilot landed the aircraft on a freeway after the engine quit because of fuel starvation. Higher than normal winds; turbulence and low altitude flying increased the aircraft's fuel consumption to higher than expected.

Narrative

Fuel starvation resulting in a landing on the interstate. Flight planning indicated arrival in 3.8 hours on 40 gallons (usable)in a Cessna 172. By the endurance profile in the POH; the aircraft properly leaned at the elevation flown should fly 3.8 hours + 45 min. reserve. Experience over 800 hours flying this aircraft show a fuel consumption between 9 and 10 GPH or 4.0 to 4.2 hours endurance. Flight conditions were strong headwinds; moderate turbulence; minimal climb-outs; and lower than typical cruise elevations. Short climb-outs would use less than the average fuel consumption. Lower cruise elevations would use more fuel (richer mixture). I didn't think of it at the time; but turbulence would result in climb-out type conditions through part of what I considered 'cruise.' Throttle and leaning procedures followed those of previous flights - 1/2 turn rich of max RPM; 2500 RPM cruise. The engine stopped at 3.8 hours elapsed on the Hobbs meter; and would not restart in the air. I was a 10-minute descent short of the planned destination. After an emergency landing on the interstate; all three sump points on the aircraft had no more than dribbles of fuel. A Mechanic completed a through review of the aircraft. Upon fueling; the aircraft started and ran. No one was hurt; nor was property damaged. The plane and pilot are both fine. There were no passengers at the time of the emergency landing. Misleading Expectations: I have rarely in the past flown for such a long duration at such a low elevation where fuel burn is higher (richer mix; slower travel through denser air). I had flow this length route regularly; but never with such consistent headwinds. I thought the minimal high-fuel-burn climb out during this day of flying would keep me well away from the high end of my historical fuel use rate in this aircraft. The fuel endurance range of the aircraft appears to have been shortened significantly by numerous small items; not any single large culprit. Human factors: For about 3.5 hours of this day's flying; I was in moderate turbulence. A portion of this with a passenger that had a history of motion discomfort. I felt pretty bad about bouncing him around. The turbulence was more taxiing on me than I thought it was. The incident was preceded by two long days; working and flying. Although; I thought I was near the top of my game at the time; in retrospect; being tired contributed to less than optimum judgment. I looked at stopping short of my intended final destination; but didn't want to leave the company plane parked outside on the ramp for a day or two in what appeared to be approaching bad weather. Aircraft issues: The left fuel gauge in this Cessna 172 has never been dependable below a = tank. I have asked a number of mechanics about it; and basically been told; that is just how the Cessna gauges [don't] work. The aircraft has a powerflow exhaust rather than issuing replacement tables for the POH; powerflow was allowed to simply placard the dash with a statement that fuel usage may be changed by the non-factory exhaust. I have developed a range of gallons per hours used on previous tanks of fuel; and use these as a rule-of-thumb for flight planning. I have flown many trips of this distance and greater; but none at this low elevation for this duration. The facts show that the rate of fuel burn was significantly higher than any of my previous experience would indicate possible. There may have been problems with the Hobbs meter counting slow. Additional checking will be completed; but is not done at this time. The elapsed tach time exceeded the elapsed Hobbs time for this trip by about 5%. Previous to this trip; the elapsed tach hours have always been less than the elapsed Hobbs hours.