[DavidG]

Well, if no fumes can bypass the filler cap in the cab, the in-cab original tank is vented at the bottom where the fumes can escape and at that point it is a matter of how well the cab is sealed off. There's webbing around the bottom perimeter of the tank, so no fumes there. There are weatherstrips at the bottom of the doors and windlacing on the other three sides sealing those areas and properly sealed floorboards as original eliminates those potential fume entry points. Of course with the windows open at idle there will be fumes no matter where the gas tank is located if it is overfilled.