I have a lancer GLX 1600. petrol I did convert it to cng. The thing to remember is there are many factors that play a part in getting better mileage out of your petrol/diesel/cng cars. May I suggest you use a good engine oil as well. Go for Micking Motor Oils. They are new in the market . It is a Korean brand. I am suing their MICKING MAX 5W/30 Fully Synthetic in my Lancer with a drain interval of 8,000kms. For your CNG Lancer may i suggest their MICKING MAX 10W/40 Fully Synthetic Motor Oil. Staer t with a smaller drain interval and then increase your way up. All new oils have different formulations so need a couple of oil changes to settle in. My Lancer averages on Petrol 12-13 Kms. Which in technical terms is suppose to be pretty good!