That's why you keep track over several tanks of gas. The light will come on
with the same amount of fuel remaining, the difference in how you drive will
only alter the miles when the light comes on.

If you are humming along on the freeway and the light comes on, you have 4
gallons at 20 mpg, but if you are dragging your boat across the landscape
and the light comes on, you have 4 gallons at 15 mpg.

It's a good rule of thumb to get gas as soon as is practical after the light
comes on.

I know what you're saying, but it doesn't satisfy my curiosity about the
warning light setting and the sensor's accuracy.  Besides, my mileage
varies.  Sometimes I'm driving around town.  Sometimes on the freeway.
Sometimes I'm towing a small boat.  Sometimes I'm towing a 25 ft trailer.

I do understand your suggestion.  And thanks.

NO!

You do not have to brave out anything. Fill the tank three or four times IN
A ROW, and caluclate the MPG, then multily your MPG by the total fuel load
that you can carry. This will tell you the total range. Using your Trip
Meter, you will find that the Low Fuel indicator comes on with roughly 3 to
5 gallons remaining. There is nothing to "tough out" because since you do
not know the unknown, you can caluclate the unknown based on knowns.

You know the MPG (or can find it easily). And you know the total capacity of
the tank. With these two known conditions, you can figure out when the Low
Fuel light will be coming on, and how far you can go before you have to
start walking ...

I'll have to brave it out and wait until the light stays on.  I've always
filled up when the warning light can't decide whether to stay on or off.

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