and at about 4 miles per litre (as you mentioned before), that will be a quarter of the rates you calculated above, say 1/3 of a penny per mile.
Todate I have averaged 26MPG on petrol and 16p per mile, over 20,000 miles in 12 months.
If LPG halves my costs, that is 8p per mile, plus 1/3p for flashlube, that is nicely below your estimate of 10 p per mile.
I like the sound of LPG even more now.
If a conversion costs me 2,000 and I save 6ppm, the conversion will break even after 33,333 miles or 1 and 2/3 years.