Originally Posted by ImmortalZ
You can calculate it pretty easily and accurately.
Tank up to full cut off, reset trip meter, drive around, tank up again to cut off. Find out how much fuel went in and divide the trip meter figure with this number to get a mileage figure. Fill from the same pump and same vending machine for both fills and you should get a figure that's about as accurate as possible.
Yup, that's how I do calculate the FE. May not be 100% accurate, but it's close enough to give me a good idea. Whatever kpl figure you arrive at, keep a +/- 0.5 margin.
Discovered this thread today. This behaviour is consistent across most cars I've known - the warning light comes on when there is a good 5-10L of fuel left in the tank. And I believe the tank volume in the spec sheet includes the reserve capacity.