I've never driven a LPG Commodore, but I did have a BF Falcon wagon (work owned, not mine) from 2006 to 2009. Mainly it was a city car (Sydney), commuting to work and driving around city and suburbs. Fuel consumption when it was new was about 20 litres per 100 Km, after a while it settled down to 19 litres per 100 KM. For Commodore comparison my previous work car was a VY wagon, normal petrol ecotec Exec 3.8, it's typical fuel consumption was about 15 litres per 100Km so the LPG Falcon was about 4 litres per 100Km more than the petrol VY exec wagon. The Falcon wagon was dedicated LPG - not petrol whatsoever. Occasionally I'd venture out of the city to a country job. One occasion I remember travelling on Great Western Highway towards Lithgow going up a bit of a hill, the Falcon was no power house. Yes sure my Falcon was loaded with work stuff but I was passed by a Hyundai Getz. The Falcon didn't kick down so would have in 4th gear, if I would have planted it sure it would have kicked down, the Getz by the way didn't even have a 1.6 badge on it, so I figure it was a 1.4
Outside the City I did do a few trips to Queensland. I'd leave Sydney with it's twin 58 litre LPG tanks full (116 litres). I did that trip 2 or 3 times in the Falcon and it would consistently make it to Tweed Heads every time and I'd fill it with 100 litres of gas every time - distance Sydney to Tweed is 800 Kms so fuel consumption 12.5 litres per 100 Km. By comparison my VS Exec at the time also VX calais and VX Clubby would all do the same trip using 10litres er 100 Km, I also had a VE SV6 SIDI, fuel consumption Sydney to Tweed low 8's and my VESS Sydney to Tweed high 8's just under 9 litres per 100Km.