My MPG

[Fazer Jake]

i usually averaged 45-50 and i now know why    previously i was filling the tank right to the brim. As it was on the stand/sloshing around fuel was leaking through the overflow pipes onto the airfilter.
 
Since filling up correctly i'm getting 55mpg

[JZS 600]

Interesting point, I always fill mine up to the bottom of the filler tube buit that's usually on the way to work so I'll have used a few liters by the time I get there..

[CRH]

nearly same as you jake?...i average 48 to 55 !...55 being on long run! never dropped below 48 though around town etc?

[DryRob]

that might help explain a few things, not forgetting my binding brakes, filthy air filter and unbalanced carbs.

[jason1]

Good tip , always filled to full and just changed airfilter and stunk of fuel. No more necking for me  Also get around the 48 mark , but mostly mid 50"s as mostly motorway miles

[JZS 600]

Filled up last night after 2 commutes to work
 
119 miles, 8.38 liters of fuel,,  64.6 MPG apparently

[Dead Eye]

I do all sorts of riding but mostly around A rounds and short stints of dual carriageway. I don't really hold back as I love riding the bike and I still manage to always be in the 50's for MPG. Most recently I got 54.2 MPG and that was after having a lot of fun on the bristol ring road But sshh, no one say anything ^^

[Fazerider]

It may be that the later model is different, but my understanding of the tank on the '98 version at least is that there are three pipes attached to it. There is the fuel outlet to the carbs via fuel filter/pump and two others, one of which drains the rainwater from around the filler, the other connects via the filler cap and is the breather. The drain and breather connect to two rubber tubes that are routed down behind the engine to the outside world (I think more recent models use a funnel rather than directly connecting to the tank). So, even if you do overfill the tank, I can't see how fuel can end up on the air filter rather than the ground.
The crankcase and carb breathers do terminate in the airbox. Is it possible that, rather than anything to do with the tank, you've a problem with a stuck float?