Maruti CNG me 32 km / kg ki claim karti he Ye toh Petrol me Itni de rahi he, first Check Petrol tank I think air Aja rahi hogi tank me may be it's possible
Mileage aise check karenge to confusion hoga hi. Hamesha autocut tak fill up karna chahie. Autocut ke upar fillup karne se fuel system bhi kharab hota hai. Minimum half tank hone ke baad hi fir refuel karein autocut tak aur mileage check karein.
To clear this confusion you could have waited for a while, may be due to air pocket in fuel, could have removed by fast acceleration and sudden braking