2001 4.6 Vogue, Petrol/LPG
After fitting the new engine at start of 2017 (core plugs were leaking - no overheating or any of the engine 'write off' scenarios people attribute with V8's - especially on LPG!!) I did the first 1000 miles on petrol to run it all in. Saw 22mpg on the dash after a run down to London. It then settled at around 18mpg on the dash over about 8000 miles.
Got the gas all working and set up, and a brim to brim on LPG (as it runs on that most of the time) worked out about 16mpg, combined. Though I can get up to about 250/260 miles out of 75L of LPG if I'm on a long trip.
It got reset when I had the transmission overhauled (again, nothing really wrong, more preventative as it's my only vehicle) and after about 1000 miles it's back up to mid 17's on the dash from memory. Though have been doing a lot more shorter/around town driving.
I also run it on A/T tyres which probably aren't the most amazing for fuel economy - but with LPG at about 65p/L and the occasional fill of petrol to keep that side all running happy (plus it starts on petrol and auto changes to gas when the coolant has hit 35 deg) - it's still pretty reasonable to run!