I don't think it's a good idea to ask what somebody else's truck does when it's your own Low Fuel Light that's on.
What if I told you I can go 175 miles after the Light comes on, how does that help you?
You know you have a 25 gallon tank.
When the Light comes on pull into a station and "Fill Up".
Your tank now has 25 gallons.
But wait! Look at the pump. How many gallons did you put in?
25(Full Tank) - (whatever the pump says) = (the number of gallons in your
tank when the light came on)
Reset you trip odometer before you leave the pump.
The next time you fill up.
Divide the miles on the trip odometer by the number of gallons you purchased.
(this only works if you fill up)
The answer is your MPG.
Now you know two things.
How many gallons are left when the light comes on
and how many miles you can go because you know your mpg.
Afterall your first question was, "how many miles can a truck go after the light comes on?
The only people who truely know the answer to that are those who have run out of gas.