when i fill the gas tank up i also put a £10 of petrol in,fuel tank is 90 litre and i ain`t filled it yet althought it`s getting there

from stone cold it turns to lpg after about 1.5 miles then runs solely on lpg.so i don`t use must petrol at all
when it`s warm it changes over almost straight away.

concerning your 20mpg on petrol,haven`t a clue mate,sorry
it`s all down to how you drive it,auto/manual,do you do town driving or motorway.

i have a mixed route really,town,dual cabbage ways and motorways.

it is only the really ideal way to run a big engined car

hope this helps

dave