Narrative:

I was returning from ZZZ1 after 4 circuits of the pattern doing full stop taxi backs in aircraft X.I pre-flighted the aircraft at ZZZ for the [short] flight to ZZZ1. The fuel gauges read tanks on the right & slightly less on the left. This aircraft has the extended range tanks. I also fly a C170B with standard tanks. When I used the dip stick to measure fuel the indication showed that I had .5 inches in each tank. In the C170B that is calibrated to 4 gallons usable in each tank. That equates to 1 hour flight time. The additional 10 gals in each tank from the size change equates to an additional 1+ hour flight time. With 2+ hours of fuel I launched without adding more.during the [short] trip to ZZZ1; I noticed that the fuel gauges were erratic; going from tanks to empty and back. So; I paid special attention to the clock to insure I could make it back to ZZZ with fuel.after 4 pattern circuits with no prolonged waiting for takeoff; I headed back to ZZZ. On the climb out I did notice that both fuel gauges were reading empty. I attributed it to the erratic gauge issue earlier as the time said I had enough fuel. I got to approximately the 6-statute mile 45 entry to xxr pattern at ZZZ. As I was descending from 2500 feet to pattern altitude; I felt the loss of power. I verified the mixture was full rich; tank selector was on both; I pushed the throttle full in and there was no increase in engine RPM. I slowed to best glide speed and [requested priority handling].I let them know that I may not make the airport and may have to set it down on [a road] that parallels the runways on the east side; closer to me from where I was coming from. As I got closer; I thought I might make xyl; the opposite to the current flight operations. As I got closer still; I realized I could not make that and set up for a landing on the road. Then I thought about trying one more thing; pumping the throttle one more time. I got a burst of power that allowed me get over airport property. But the hangars I was now over put me too high for a xyl landing; I continued across the field to the xyr runway and managed to set the airplane down in the last 1;000 feet. And get stopped before running off the pavement. I rolled off of xyr onto the run up area between the runways. I pumped the throttle several more time to keep the engine running and got permission to cross the takeoff end of xxr to maybe get back to the FBO; but the engine quit while on xxr and I had to be towed back to the FBO. Total time for the flight was 1.5 hours hobbs.what I learned was that 1+1 does not necessarily equal 2+ when you do not know exactly how 2 different sized tanks relate to a calibrated dip stick for the smaller tank.also learned not to pass up 4 options to put gas in the tank. 1st before takeoff; 2nd at ZZZ1 before returning; 3rd at ZZZ2 going and coming back.

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Original NASA ASRS Text

Title: A Cessna 170 pilot reported undercalculating the fuel required for a flight; resulting in a fuel starvation event; and a near off-airport forced landing.

Narrative: I was returning from ZZZ1 after 4 circuits of the pattern doing full stop taxi backs in Aircraft X.I pre-flighted the aircraft at ZZZ for the [short] flight to ZZZ1. The fuel gauges read tanks on the right & slightly less on the left. This aircraft has the extended range tanks. I also fly a C170B with standard tanks. When I used the dip stick to measure fuel the indication showed that I had .5 inches in each tank. In the C170B that is calibrated to 4 gallons usable in each tank. That equates to 1 hour flight time. The additional 10 gals in each tank from the size change equates to an additional 1+ hour flight time. With 2+ hours of fuel I launched without adding more.During the [short] trip to ZZZ1; I noticed that the fuel gauges were erratic; going from tanks to empty and back. So; I paid special attention to the clock to insure I could make it back to ZZZ with fuel.After 4 pattern circuits with no prolonged waiting for takeoff; I headed back to ZZZ. On the climb out I did notice that both fuel gauges were reading empty. I attributed it to the erratic gauge issue earlier as the time said I had enough fuel. I got to approximately the 6-statute mile 45 entry to XXR pattern at ZZZ. As I was descending from 2500 feet to pattern altitude; I felt the loss of power. I verified the mixture was full rich; tank selector was on both; I pushed the throttle full in and there was no increase in engine RPM. I slowed to best glide speed and [requested priority handling].I let them know that I may not make the airport and may have to set it down on [a road] that parallels the runways on the east side; closer to me from where I was coming from. As I got closer; I thought I might make XYL; the opposite to the current flight operations. As I got closer still; I realized I could not make that and set up for a landing on the road. Then I thought about trying one more thing; pumping the throttle one more time. I got a burst of power that allowed me get over airport property. But the hangars I was now over put me too high for a XYL landing; I continued across the field to the XYR runway and managed to set the airplane down in the last 1;000 feet. And get stopped before running off the pavement. I rolled off of XYR onto the run up area between the runways. I pumped the throttle several more time to keep the engine running and got permission to cross the takeoff end of XXR to maybe get back to the FBO; but the engine quit while on XXR and I had to be towed back to the FBO. Total time for the flight was 1.5 hours Hobbs.What I learned was that 1+1 does not necessarily equal 2+ when you do not know exactly how 2 different sized tanks relate to a calibrated dip stick for the smaller tank.Also learned not to pass up 4 options to put gas in the tank. 1st before takeoff; 2nd at ZZZ1 before returning; 3rd at ZZZ2 going and coming back.

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.