read this thread this is very informative and i hope your question will be answered.
This is understood that you cant drive at 50 KMPH on highways, but ur bike will give you better average on higway even if you drive it at 70 - 75 KMPH. Reason is that you drive consistently at a speed on highways, it means less usuage of breaks and less shifting of gears. thats what saves the fuel.
By the way what average you are getting from your 125 in city.