I had a LPG xr4i, much better on lpg than petrol(and faster), assume it must be due to the hopeless siamese ported std heads, I had a Tartarini tech 97 with a wolf n 3d spark advance unit, running massive advance on lpg, 110 litre tank in rear got around 28mpg on lpg and around 22mpg on petrol, was filling up at a mates(pig) farm where lpg used for heating so was 23p a litre(a few years ago, maybe 6?).
tabetha