As I am looking at the problem here are my 2cents..
Running a bike uptill 8k in 5th gear means sealing it on 90kmh ... and if not the the top gear and using lower gears more fuel is consumed, so Ahsen bhai do you really think it would do any better with reving engine high?
I am using a old piaggio with worn out engine it gives me around 35~38 and in straight roads Max 40km/L, but please do keep in mind that I am using a Fat heavy Bike Tyre and with free Flow Heavy Bike exhaust and Free Flow air Intake so it justifies why it is doing under 40km whhile in her glory Days it reached 50km happily under 6k-RPM where as Ravi Claims it to even reach 60KM/L
Ahsen bhai one more thing I wana share : The Bad Thing which ever Happend in 2009 was that My bike Belonged to the Imported Lot which had a Fuality Carb and a Fuality EuroII device which did that "patakha" thing on reving high (just like new CGs) and mileage was bad and bike was not any way Idling properly I read in news paper that Ravi has called in all Paiggios to get the Carb and Euro-II device replaced for free and I got it changed and after that no problem ever faced related to Carb or EuroII Device ... last tuning was done 6 months back as I dont have any filters to clean in bike and paiggio has self adjusting valves so nothing left for tuning only oil changes are done..
get little freindly with your ride notice closely you will identify what makes her consume more fuel ... are you haevy wieght and ride with pillion rider, or do you rev it and lead in front on all cars bikes when ever the signal light turns green on roads.. e.t.c.. and you will identify the problem soon...
Omer85 did a speedy sprint from Islamabad to lahore when he reached Lahore we did some rough calculations that what mileage he achieved on high speeds and it was roughly 27~35 KM/L if that very same tour was done on lower speeds results have been different so I hope you I explained it well.