Q. What should be the average speed for getting good mileage for Yamaha fz?

There are multiple factors that affect the mileage of any vehicle such as riding style, riding conditions, tyre air pressure, periodic maintenance, etc. In order to get better mileage, riding your bike at economical speed also helps but you need to keep in mind the aforementioned factors as well. In order to get better fuel efficiency returns from your bike, there are some points that you can keep in mind while riding such as keeping a check on tyre air pressure, timely service your bike, maintain proper gear to RPM ratio, do not accelerate the bike unnecessarily, ride your bike at economical speeds (50-60 kmph), etc.

You can maintain 70kmph in 5th gear it gives very good mileage, average speed is 40

60-80