Narrative:

I was conducting a student's first cross country flight from ZZZ to ZZZ1 in a cessna 150. My student preformed a pre-flight and I checked the fuel and oil quantity. Using the dipstick provided with the plane we both determined the plane had a total of 15 gallons; and at 6 gph this would provide 2.5 hours of flight; more than enough to make the 50 NM flight and return. When we got to ZZZ1 the wind conditions were conducive to practicing cross wind landings and we performed a full stop landing and two touch and goes.we headed back to ZZZ and on the way discussed engine failure and what to do if that were to happen. We used flight following to transit [other] airspace. Once the airport was in sight and we had the weather flight following was canceled and we were switched to advisory frequency. On a straight in approach at 700 feet the engine quit. I took controls and determined that the plane would not make it to the runway. We landed on the highway a quarter mile from the airport. There were no injuries and no damage to property. Because the plane was low on fuel and the intake was located towards the back of the wing; when we pitched down for our descent; fuel could not reach the intake. In my training at a different flight school; fuel dipsticks were calibrated to show usable fuel. The dipstick provided with the plane I was flying showed total fuel and we were supposed to factor in the unusable portion. I was unaware of this.on final; the engine became rough. I asked my student 'what are you doing?' and he said 'I'm not doing that'. I took controls of the plane and found that the throttle was not responsive and at about 700 feet AGL the engine stopped.I pitched for glide. Determined we wouldn't be able to make the runway and chose the road as our best place to land. I made sure carb heat was on; turned off the fuel shutoff; mixture full lean; key out; master off; and landed it.as PIC (pilot in command) I take full responsibility for the flight. My honest opinion is that it was the result of unintentional fuel starvation. I misunderstood what the reading on the fuel dipstick was telling me. I also could've scanned the fuel gauges more thoroughly.I was concerned about staying underweight in the cessna 150; otherwise I would have used full fuel. Because I thought that the plane had 3.5 gallons more fuel than it actually did; I thought I had 40 more minutes of reserve. This was in fact the unusable portion of fuel for a cessna 150. I should have refueled the plane at ZZZ1.I wanted my student to save money by continuing training in the C150 as opposed to the 172. Because we had a smaller plane; the fuel we could carry was limited. I have read several publications on the fuel system of the cessna 150 since the incident. I want to learn as much as possible; so that this never happens again. The flight school that I work for now explicitly details the fueling requirements to include the unusable portion. I will now consider whether a fuel stick is calibrated to show unusable and usable or just the usable portion of fuel.

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Original NASA ASRS Text

Title: C150 flight instructor reported an off field landing due to fuel starvation on short final.

Narrative: I was conducting a student's first cross country flight from ZZZ to ZZZ1 in a Cessna 150. My student preformed a pre-flight and I checked the fuel and oil quantity. Using the dipstick provided with the plane we both determined the plane had a total of 15 gallons; and at 6 GPH this would provide 2.5 hours of flight; more than enough to make the 50 NM flight and return. When we got to ZZZ1 the wind conditions were conducive to practicing cross wind landings and we performed a full stop landing and two touch and goes.We headed back to ZZZ and on the way discussed engine failure and what to do if that were to happen. We used flight following to transit [other] airspace. Once the airport was in sight and we had the weather flight following was canceled and we were switched to advisory frequency. On a straight in approach at 700 feet the engine quit. I took controls and determined that the plane would not make it to the runway. We landed on the highway a quarter mile from the airport. There were no injuries and no damage to property. Because the plane was low on fuel and the intake was located towards the back of the wing; when we pitched down for our descent; fuel could not reach the intake. In my training at a different flight school; fuel dipsticks were calibrated to show usable fuel. The dipstick provided with the plane I was flying showed total fuel and we were supposed to factor in the unusable portion. I was unaware of this.On final; the engine became rough. I asked my student 'What are you doing?' and he said 'I'm not doing that'. I took controls of the plane and found that the throttle was not responsive and at about 700 feet AGL the engine stopped.I pitched for glide. Determined we wouldn't be able to make the runway and chose the road as our best place to land. I made sure carb heat was on; turned off the fuel shutoff; mixture full lean; key out; master off; and landed it.As PIC (pilot in command) I take full responsibility for the flight. My honest opinion is that it was the result of unintentional fuel starvation. I misunderstood what the reading on the fuel dipstick was telling me. I also could've scanned the fuel gauges more thoroughly.I was concerned about staying underweight in the Cessna 150; otherwise I would have used full fuel. Because I thought that the plane had 3.5 gallons more fuel than it actually did; I thought I had 40 more minutes of reserve. This was in fact the unusable portion of fuel for a Cessna 150. I should have refueled the plane at ZZZ1.I wanted my student to save money by continuing training in the C150 as opposed to the 172. Because we had a smaller plane; the fuel we could carry was limited. I have read several publications on the fuel system of the Cessna 150 since the incident. I want to learn as much as possible; so that this never happens again. The flight school that I work for now explicitly details the fueling requirements to include the unusable portion. I will now consider whether a fuel stick is calibrated to show unusable and usable or just the usable portion of fuel.

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.