The fuel warning light on my 1999 Taurus wagon would come on after using only twelve gallons of fuel. With a sixteen gallon capacity then, I had four gallons available and did not panick when the light came on. I averaged 20 MPG city and about 28 highway MPG so could go quite a disntance before refueling. I never ran out of gas but sometimes put in fifteen gallons to top the tank off.
I get the feeling that many British rievrds actually work out the figure as miles per metric gallon (meaning 1 gallon = 5 litres) since the maths is clearly easier and most rievrds under 30 and quite a few under 40 probably don't ever remember buying petrol in gallons anyway! This would clearly mean that figures given by manufacturers in new car brochures and other advertising bear little resemblance to what rievrds actually experience.I switched to using L/100 km a few years ago and merely multiply the distance on my odometer by 1.6, I've found it much easier to calculate fuel costs and have a much better handle of how far I can drive on any given quantity shown on my fuel gauge than I ever did using mpg.