I believe the manual specifies that the light comes on at 1.3 u.s. gallons remaining... IF its accurate, it implies a good 20-25 miles of highway driving (depending on the typical factors, weight, type of driving, etc.). I usually find something within 10 miles, because, well, its a Lotus.I don't have the manual readily available, but I believe the the light should go on when there's a specific amount of gas remaining? One would think it'd be at least a gallon or more.
The tank holds 10.6 Gallons (or 40 litres)