Um, reserve of 5 gallons at 18mpg is like 90 miles. How far do you want to go after the light comes on?

So while it DOES have a 20 gallon tank (absolute volume), the light will come on once you hit the reserve (and it will show empty at this point) or the "usable" volume, if you want to call it that. It's their way of trying to keep you from running out completely and being stranded. My '03 will only go (to the best of my knowledge) about 35-40 miles. At least that's all the further I've pushed it.