Splitting hairs even finer here....
The anomalous expansion of water (with cooling) occurs from 4deg C to 0 deg C
Avgas always expands with increasing temperature (and vice-versa). Density is generally quoted at 15 deg C (Metric system) or Specific Gravity at 60 deg F (imperial system)
Therefore to accurately determine weight, you need to measure volume, temperature, and Density/SG. Then apply a correction for buoyancy. (-0.0011 g/cc) for density) Easy isnt it!!!
Simple answer:
1 US gallon weighs 6lb
1 Imperial gallon weighs 7lb