Running dry?

"Neil Gould" wrote:

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.

Let's examine that. We're comparing the relative risk of two events.

Event 1 is that the fuel selector valve fails when you go to switch to the
tank containing the remaining 8 gallons.

Event 2 is that your estimate of how much fuel is left in the tank is wrong
by 4 gallons (16%).

Which is more likely?