I don't know for certain, but the vehicles that I have owned with this light
seem to carry about 3 ~ 4 gallons when the light comes on.

You can figure this out though.

Fill the tank and notate the mileage. The next time you need gas, FILL the
tank again and notate the mileage again. Subtract the bigninning number from
the ending number, and divide that by the number of gallons you buy. The
result is your average mileage for the type of driving you do.

Repeat this a few times to get an average over a longer term. (Add all of
the fuel mileage numbers together and divide by the number if figures you
added together, ie. 15.9 +14.6 + 16.6 = 47.1 / 3 = 15.7)

Now, you will have found that you can get about 15.7 mpg, and you already
know the truck has a <nn> gallon tank (let's just say 25 gallons). This says
you should be able to go about 390 miles on a tank, if the light comes on at
345 miles, then you still have about 3 gallons (15 x 3 = 45 + 345 = 390) in
the tank when the light comes on.

You need to do the math, but simply substitute your actual numbers for my
pretend numbers and you should come up with something in the range of 4
gallons remaining when the light comes on.

I checked my owner's manual (don't tell anyone) and it only says the warning
light signifies low fuel.

How many gallons are left in my tank when the low fuel warning light comes
on?

2001 F-150 Supercrew