Just out of curiosity how many miles per tank do you get from a4.6 ?
All are different,i have a 4.6 she has done 57k(48k when i had her 5 years back)i do very low mileage but around town get 12-14 on a main road with cars moving about 19-20 and at 60-65mph i get 25-26.As i dont do much mileage changing to LPG is not worth it,but thats just me if you do say 10k a year then things change.