C172 pilot reported landing safely on a highway after running out of fuel.

Narrative:

Lost engine power possibly due to fuel exhaustion near the end of a VFR cross country flight. Landed safely on [a highway] without harm to the pilot and 3 passengers or people on the ground.the aircraft was fully fueled to 50 gallons before departing. After 3.5 hours to [destination]; the pilot had 24 gallons of 100 ll added at the FBO. Due to the aircraft being close to gross weight; it was not topped off. Total flight time that day was 7 hours with a total of 74 gallons available.the C172 has an stc that installed a lycoming 180 hp engine. The pilot discussed with the owner the fuel consumption planning number as 8.1 gph and had seen that performance on previous flights of the aircraft; mostly solo flights. The stc paperwork was provided by the owner to the pilot for computing the weight and balance for the flights. The stc increases the max takeoff weight to 2550 pounds. The pilot loaded the stc weight and balance values into his garmin pilot app and used it to compute the weight and balance. The stc paperwork did not include changes to the aircraft endurance; climb; or other performance changes from an unmodified C172.the pilot used 8.1 gph for planning the flights thinking that he had a total endurance of over 9 hours for both flights. Not completely knowing how the aircraft would perform with the maximum payload may have allowed the pilot to underestimate the fuel that would be consumed. Perhaps stcs should more fully describe any impacts to the aircraft performance.

Original NASA ASRS Text

Title: C172 pilot reported landing safely on a highway after running out of fuel.

Narrative: Lost engine power possibly due to fuel exhaustion near the end of a VFR cross country flight. Landed safely on [a highway] without harm to the pilot and 3 passengers or people on the ground.The aircraft was fully fueled to 50 gallons before departing. After 3.5 hours to [destination]; the pilot had 24 gallons of 100 LL added at the FBO. Due to the aircraft being close to gross weight; it was not topped off. Total flight time that day was 7 hours with a total of 74 gallons available.The C172 has an STC that installed a Lycoming 180 hp engine. The pilot discussed with the owner the fuel consumption planning number as 8.1 gph and had seen that performance on previous flights of the aircraft; mostly solo flights. The STC paperwork was provided by the owner to the pilot for computing the weight and balance for the flights. The STC increases the max takeoff weight to 2550 pounds. The pilot loaded the STC weight and balance values into his Garmin Pilot app and used it to compute the weight and balance. The STC paperwork did not include changes to the aircraft endurance; climb; or other performance changes from an unmodified C172.The pilot used 8.1 GPH for planning the flights thinking that he had a total endurance of over 9 hours for both flights. Not completely knowing how the aircraft would perform with the maximum payload may have allowed the pilot to underestimate the fuel that would be consumed. Perhaps STCs should more fully describe any impacts to the aircraft performance.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	1566169
Time
Date	201808
Local Time Of Day	1801-2400
Place
Locale Reference	ZZZ.Airport
State Reference	US
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	Skyhawk 172/Cutlass 172
Operating Under FAR Part	Part 91
Flight Phase	Cruise
Route In Use	Direct
Flight Plan	None
Component
Aircraft Component	Engine
Person 1
Function	Pilot Flying Single Pilot
Qualification	Flight Crew Commercial Flight Crew Instrument
Experience	Flight Crew Last 90 Days 16 Flight Crew Total 818 Flight Crew Type 818
Events
Anomaly	Aircraft Equipment Problem Critical Inflight Event / Encounter Fuel Issue