Mitsubishi Pajero 1997: LPG fuel consumption

The LPG consumption is very good, almost too good to believe, and the petrol consumption is woeful. I would expect that you should be able to get 17.5L/100 km or so on LPG, but you're getting much better than that, and about 14 L/100 km on petrol and you're doing much worse than that. I would take it to another LPG installer, one that is familiar with the system fitted to your car and have them thoroughly check it on both fuels.