At sealevel 360 fuel injected engine, full throttle full rich on takeoff I
see about 17-18 GPH.  You should test your gravity flow with the plane
inclined on a hill or on hill and blocks at your climbout angle and do a
gravity flow from your tank.  You should get about 50% more gravity flow
rate than 17-18 gph or about 25-27 gph flow rate.

Bob Smiley
N94RJ