400 Kilos = 250 miles.  Mmmm, I regularly go 240 miles and put in about 4 gallons after hitting reserve, occasionally it goes 250-260 miles on the 4 gallons. 

Why not just check the lines to make sure they are right?

http://www.gs500.net/gallery/data/500/GS500fuelcocks.jpg

So the pipe at bottom right is the reserve following it to the tank. My bike is an 04. Is that correct?

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"Bottom right" is a little ambiguous in the photo, but yes ... the hose from the front outlet tube under the tank (which should be the longer of the two) should connect to the rear-most inlet on the frame-mounted "fuel selector switch" (petcock with ON / RES / PRI settings).

Yes, like Kerry just said.  Here's my diagram for 04-09 models:

It's kinda hard to tell so the line at the bottom is connected to my reserve on the tank and the one on the right is connect to on.

Maybe this will clear things up?

Good thinking Kerry!

This petcock has got to be the most misunderstood part of this bike, I swear!

I think mine are the wrong way around then as the cable at the bottom of the petcock goes to the front part of the tank pet rock and the house on the right goes to the one at the back. Which is in the right when looking at the frame petcock

I have swapped them around ill go for a ride tomorrow to see what happens

On the tank petcock, there is an "R" on the 1990 model embossed in the body of the petcock, the long one RESERVE.

Yeah they were the wrong way around, cheers