Honda SLR Fuel Tank
 

I have a Honda SLR 650 and while it is not a true off-roader it can cope. More particularly it is good on tarmac which is mostly what I do.
The problem is the size of the fuel tank (about 13L incl. reserve. I usually get about 110 miles before turning on to reserve and thay's pathetic!
Does anyone know the best way to increase capacity. Acerbis do plastic tanks for the XRL 650 but would it fit? Would I need a new seat?
Does anyone know about these bikes/

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Acerbis also make a 23 litre tank for the Dominator 650, which should fit straight forward since your bike is a stripped down version of that model.