Mehran Not Picking the Speed

[tahahussain1995.th1]

I Have Mehran 2007 VXR (CNG). Few months back I repaired its head as it was consuming oil. After that my mehran is not going above 80km/h on CNG and petrol both before head repair it touch hundred on CNG with AC ON. Can someone tell me what to do now and how can this issue be resolved.
Secondly fuel average is so bad on petrol it gives me around 7 to 8 on petrol and on CNG average is 15 to 18.
How to get good fuel average on petrol

[aziz.rehan]

Is fan direct?
Is thermostat valve installed and working properly?
Where does heat gauge stays after starting car for few minutes
Check spark plugs,fuel and air filter
Which oil and grade u use?

[tahahussain1995.th1]

1) Fan is not direct
2) Needs to check (but heater does not work because mechanic ny wapsi pipe engine ma daal dia)
3) In Middle
4) Already changed two times
5) 20 50

[aziz.rehan]

Do a compression test to check health of engine