Run it until the low fuel light comes on, fill it up and make a note of how much fuel you put in, convert this to gallons, reset the mpg avg on your computer and reset trip A to zero on your speedo. When the fuel light comes on again fill it up and note the amount of fuel in gallons again and the miles covered on trip A, do the sums and compare to your mpg avg. As a rough guide, the Zed holds approx 18 gallons of fuel so 18 x 20mpg = 360 to tankful or 18 x 25 = 450 tankful. Keep in mind though that the amount of fuel left can vary from car to car when the low fuel light comes on. Mine used to be about 3 gallons, I've done the fuel gauge float clean thing and it's now about 2 gallons. It's a ball ache of a job and as ionabee says not the safest thing to be messing around with. Alternatively you could buy one of these:-
http://www.ebay.co.uk/sch/i.html?_nkw=s ... angauge+ii