The reserve light comes on when there is about 1.1 gals. left in the tank, (at least mine always does) if you are on level ground. After my first oil change I got 408 miles when my light came on...thats 30.4 mpg. In combined driving I get 26 mph. At that figure you have about 28 miles on the reserve. When my needle is ALL the way down past the -R- I am bone dry. My gage seems to be metered farily presise. I have 5500+ miles on mine and it has been very consistant.