I know rpm nhi hota, that is why asked you how much time have you been driving for. Us se bande ko andaza ho jata hai kitne rpms pr gaari chal rhi hai. For a 660cc car on khi roads, my guess is gaari 60 ki speed se oper jaegi tou 2.5k rpms cross krjaegi. Means shahrah e faisal pr bhi aap ne 60 se oper kisi soorat nhi jana. Do it for a full tank, full to empty, then check mileage.
Boht hi vague but simple tariqa bta rha hu. Bridge pr jaate waqt 30-40 max speed hogi at 2.5k rpms