Recently, Roy Smith posted:

"Neil Gould" wrote:
What FAR says you may not run a tank dry?

The FARs address minimum fuel levels when you arrive at your
destination.

I'm not aware of any such regulation. I suspect you're thinking of:

91.151 Fuel requirements for flight in VFR conditions.

[...]
But that only talks about how much fuel you have at takeoff, not at
landing.

You're (all) right; I was thinking of that FAR, and I stretched the point
too FAR. ;-)

I regularly fly something with two tanks and no "both" position
(PA28), and my preference is to arrive at my destination with more
than 30 minutes worth of fuel, period. I see no point in pushing
those limits any more than seeing how much over gross I can fly.
IMO, such points are just useless information. YMMV.

I also think landing with 30 minutes of fuel is too little. So, how
much is enough? Let's assume we can agree on an hour, which in a 180
HP PA-28 means about 8 gallons. You take off with 48 usable and fly
for 5 hours, leaving an estimated 8 gallons left. Which is a more
useful configuration to have at this point, an estimated 4 gallons
usable remaining in each tank, or an estimated 8 gallons usable in
one tank and the other one dry?

I would feel more comfortable with 4 in each tank than with a dry tank.

I had an interesting thing happen to me in an Archer. During an XC, a
facia screw on the fuel selector had worked loose and backed out enough
that when I went to switch tanks, it wouldn't go into that position. My
choices were the tank I was on, or off. I'm glad the tank I was on wasn't
dry, because when the A&P looked it over, it took him around 15 minutes to
figure out what was wrong. Needless to say, I wouldn't have figured that
out en route before hitting the ground, because I couldn't see the problem
from my seated position. Stuff happens.

Neil