On level ground at a constant speed, weight does not have a very big effect on fuel economy. Depending on the coefficient of rolling resistance, of course. Not too long ago, I went on a road trip with 800lbs of people and stuff in my car (fat Americans xD), and averaged 33mpg US (~40mpg Imperial). I was picking up a few friends who were on vacation, so the way down there it was only me and a friend that I was splitting the driving with. The way down without all that weight, we also averaged 33mpg. In the city, more weight will hurt your fuel economy (If you insist on accelerating at the same rate as with less weight). Though, if you have enough weight it'll increase the force of rolling resistance to a noticeable degree, of course, since Frr=Crr x FN
Now, in my case, my fuel tank is about 12 US Gallons. The density of petrol is about 6.073 lb/US gal. 6 US Gallons of fuel would only weigh 36lbs. Not enough to have a huge affect on fuel economy on the highway. And probably not in the city either.