I want an hour reserve, even day VFR. Last week I landed for fuel 35 miles short of my destination, because to go on would have cut into that hour -- only by a few minutes, but that's my rule. 'Cuz there can be surprises.
Years ago my Bonanza's engine started sputtering 20 minutes before it should have, while I was running on the aux tanks. When I landed there was still plenty of fuel in the tank. Turned out an O-ring in the fuel cap was bad, and suction from lower pressure on the top of the wing prevented fuel in that tank from flowing past the checkvalve (and a big hello to
you, Dr. Bernoulli!).
Then there was the time I left the fuel cap off the right tank of my Sport Cub after refueling at McCall, Idaho. In cruise, with the fuel selector on 'both', I noticed that the right tank remained full and the
left tank was feeding much more rapidly than normal. Why? As fuel was siphoning out of the right tank, the suction from the missing cap was cross-feeding more fuel through the vent line from the left into the right. After a prompt landing I did the first smart thing all day: I bought
two new caps.
Here in the toasty desert southwest, fuelers often ask if we want them to leave a little room at the top for expansion. That might be a good idea if the airplane will be parked in the sun for a while, but you never know how much is really in there.