In message , tony sayer writes
Use propane or propane/butane mix as they do in Holland. In the Autogas conversion business the rule of thumb was that the car would do about 10% less in gas than on petrol, and a KG of propane is about 2 litres. Usual practice was a cylindrical tank (say 80 litres) in the boot or a doughnut , say 50litres in place of the spare wheel. The big advantage of using propane was not lower CO2 emission but cleaner exhaust, minimal CO, minimal unburned hydrocarbons, no SO2 and no nitrous oxides. Having encouraged conversion with lower fuel duty the government then screwed it up by starting to phase out the differential.