IO540 - that's a very professional and involved way of doing it! Do 2 flights, identical profile etc... all very good when you are an FI, or own your own a/c - much less practical when you rent a club aircraft once/twice a month! I'm not too sure most people would want to repeat flights just to assess the fuel burn. I know I wouldn't.
As for leaning the mixture, it's always highly recommended (certainly on longish trips), but there isn't much point doing it up to 3,000 ft or so; you won't get a significantly lower burn.
I totally agree though about checking the tanks, and if possible, flying with full tanks if airfield performance/mass & balance allow. As I've said before, there is hardly any performance penalty for doing so, and on a PA28 with an engine in reasonable nick, you'll have about 4.5 hours endurance. Incidentally, a new PA28-161 is meant to burn slightly less than 9 US Gallons/hr, but an older engine will use up to 20% more. I do my fuel planning using a 10 USG/hr figure (including power check, take-off and climb), and when checking tanks against fuel gauges afterwards, there is slightly more left than my calculations show. The number of different PA28-160/161 airframes I have flown over the years had a fuel burn varying between 8.5 and 10 USG/hr.
But I agree that a fuel flowmeter is ideal; I have "snagged" PA28s a couple of times due to one fuel gauge becoming u/s in flight and showing empty. If the a/c has no tendency to roll and the engine still feeds OK from that tank, it's obviously a gauge problem and not a fuel leak; having no fuel indication on one side is another good reason for proper fuel management (and to calculate fuel remaining at each waypoint).
Cheers
