50 miles? but at what pace, I can get mine down to 15mpg, what if I'm cruising at 25mpg?

 

There are 2 gallons remaining when the light comes on.

 

What? Why not just divide by the number of gallons you just filled up

 

Same for me. My light has come on at 9.8 gallons used before!

Keep in mind the S is prone to fuel pump starvation when your running near the light if you start cornering hard.

 

right. would you want the light to come on when you had 1/4 liter or 1/4 gallon left instead? no, because it might take you awhile to find a gas station.

mine seems to come on with almost exactly 2 gallons left every time.

when you run out of gas, there are way more lights on in the dash display...because the key is in the "ON" position but you can't start the car. how's that for low fuel warning??

 

If they do it by miles which they do it in my other car. Then they probably just calculated from the avg mpg city/hwy and that would about 25 = 2gals

 

American car fuel gauges have a significant fuel margin per american driving preference. In Germany (and maybe other parts of europe) the fuel margin is much less. The Germans want the fuel gauge to tell them exactly what is in the tank. The margin is adjusted to the population where the car is sold. When gas prices were high and everybody was running on empty, a tv show did a report on how far you could drive on empty, which including these observations.

 

its the 'reserve' tank ... the light is basically letting you know you have ~10L - 2gals left in the tank.

its in the owners manual.

 

Any chance it might also be a precaution to protect the engine from sediment or other crap in the bottom of the tank?

 

There is no need to extrapolate.

You simply reset one of your trip odometers when you fill up. Then on your next fill up, note the volume of fuel you are taking in and use it to divide the reading on the trip odometer.

I know people who took the pain of using the same gas pump and made sure they have their typical balance of highway and city driving to figure out their gas mileage.