PA28-180 pilot is surprised to learn he had only two total gallons of fuel on board his aircraft upon landing following a long cross country flight.

Narrative

Fuel service refilling the aircraft the day after this event reported finding approximately two gallons fuel in one tank; and the other tank dry. I had planned the flight at 2:32 enroute and expected at 10.5GPH fuel burn and departing with fuel to the tabs to have 45 minutes reserve time upon landing; the actual flight was less than 10 minutes different in elapsed time than the plan. There are several PA-181s at the flying club; it had been some time since I'd flown this particular aircraft. I had experienced difficulty in getting reliable EGT reading to guide engine leaning in the previous day's outbound flight. I had been somewhat cautious to run rich of peak due to uncertainty about 'peak' because of my difficulty with the EGT. During the return flight I was similarly cautious in leaning -- in retrospect by running rich of peak my fuel burn rate was above 10.5GPH. In planning this flight I had also failed to plan a higher fuel burn during the initial climb phase. During the return I ran one tank dry; which should have told me that my planning was off -- I had expected to leave a 10-minute reserve in that tank. I took action at that point to return by a more direct rather than the more usual return. The fuel gauge for the other tank showed between 7 and 9 gallons remaining as I passed over ZZZ1; which was the last diversion I considered before landing at ZZZ. The gauge reading and the elapsed time told me I had ample reserve. In retrospect I did not have ample reserve; and risked a serious night emergency landing or ditching in the water. I did not adequately question the experience of the tank running dry early. My decision to press on was faulty. My new figure for planning fuel burn is 12GPH for the Piper; and I will review fuel planning and leaning for the Piper with my CFI; as well as this entire flight experience; before flying again.