The fuel tank capacity is 15.3 gallons according to Alfa. The 'reserve' is all at the top of the gauge; there is no reserve at the bottom. As a result, the gauge is a bit misleading. I always gas up at 1/4 tank. If the gauge were accurate, filling up at 1/4 tank would result in about 11.48 gallons but it doesn't. It's more like 12.8 which means at 1/4 tank, you have about 2.5 gallons of gas left. At 1/8 of a tank, you have about 1.25 gallons left.
You can more accurately estimate your 'miles to empty' by knowing what your actual miles per gallon is, and multiplying that by the mark on the gauge. For example, I average about 24 mpg, so at 1/4 tank I know I have roughly 50 miles of range until empty, but that's assuming a steady driving speed. If you watch your mpg estimate on your gauge, it swings over quite a range depending on whether your accelerating or cruising. At 1/4 tank I know at best I have about 50 miles till empty. If I'm climbing a hill, I may have about 20-25 miles till empty. You don't want to run the tank empty; that can damage the fuel system. At 1/8 of a tank, at best I may have 20-25 miles till empty, at worst it could be 10-12. If you go by the estimate on the gauge, it may say you have 40 miles till empty at 1/8 of a tank but you can be sure that's not even close to accurate.
As a general rule, just start looking for a gas station when you reach 1/4 tank because in reality you only have about 2.5 gallons left at that point. Anything below that is starting to get tight. If you're down to 1/8 of a tank, you have about 1 gallon left which is emergency gas levels.
(BTW, I don't use the gauge for my mpg numbers. I always divide the miles driven by the actual number of gallons I put in the car at each fill-up.)
