My service manual says that the standard and classic R3 holds 25 liters, the owners manual says 23.5 liters (6.2 US gallons). I've never had her TOTALLY empty, but I have put 6 gallons in at one time before. Because of this I've always trusted that she does indeed hold every bit of the 6.2 and then some. I don't doubt getting 6.3 in there at all, so I wouldn't bet against it. In fact, if she were bone dry and you filled it to the brim, I think she could hold 6.5.