My q50 premium 3.7 awd The gas autonomy is 15.7 mpg on city ,before was 17.8 on city, the mustang 5.0 gt is 18.8 on city, something is wrong with my car....

Aside from a tune-up/filters change there may be nothing wrong.

I've owned 3 Infiniti's; G37 IPL, QX50 and currently a Q50 RS400, frankly mileage is not their strong point. The 3.7 is a fine engine but not known for great mileage, I was getting around 11mpg with my IPL (brisk city driving). Sounds like a trip to your dealer should be your first move............

I never check the filters, I paid a recommended maintenance in infiniti last year with 20000 miles, now I plan to pay another to 25,000 miles