Q. I run 300 km in the speed 90 km is their any problem in mileage?

Not sure if you are saying 300kms at a stretch @90kmph. Its not advisable. 300 km trip with short breaks is fine with variying speeds between 50 to 90 kmph. You should ensure good quality petrol, engine and gear box oil, right air pressure in both the tyres. when you are freaking out at 90kmph speed range, you should be thinking only about vehicle maintenance and not mileage.

We would suggest you ride slower, in order to improve the mileage. Riding fast can reduce your fuel efficiency by up to 33% if you are travelling above 60 kmph. It is best to change gears appropriately and match the gear and the RPM carefully.

You will not get better mileage at high speed. More speed less mileage it is applicable for all types vehicles. 40-55 km/hr. is the best for best milage.