@salman.ateeq919407
Sallu Bhai, read Noise rating written on your tyres. Unit will be in decibel e.g. 43 db, 52 db, 65 db.
I have also ripped off buying a famous brand which is marketing & advertising in Pakistan as "Silent" but it was noisy so when i complained they told read your noise rating; unfortunately it was 72 db. They told they are marketing it as SILENT because at least it is less than 80 db 
What the rubbish justification but it is ofcourse buyer's mistake not to read these ratings in addition to these tyre walas who usually do not educate buyer about this and consider only their profit margin. Each & every detail written on Tyre is note worthy.
According to research, any sound above than 80 db(80 decibel) is considered as Noise and if u r subjected to it continously for sometimes then Ear hearing capability will be affected
I was also shocked if new one is 72 db & with passage of time it definitely increased upto 85 or 90+ specially if u r driving on Karachi roads
I think u have bought high noise rating Tyres, if this is the case, then definitely all high noise rating tyres decrease the mileage due to their material type & structure
NOISE POLLUTION IS ALSO TYPE OF POLLUTION which most of Chinese brands are not considering hence causing high sale of High noise rating in our Country which could not be exported to many of Gulf countries. For their country, they would have strict standards but for our poor country, they are doing this
But i am surprised, as u have used the term "Drastically dropped" what it means? how much difference u r experiencing. May be some other fault comes coincidentally together related to fuel efficiency which u r mingling with Noise rating.
