what Tab said, times a bunch [years and trucks]
but some wanna put their butt on the line, so to speak, in order to gain all the "useful space" behind the seat for junque collecting
I find custom cutting a piece of wood to sit on top the tank makes for a handy "trunk" for tools and such, without the risk of a ruptured tank from some yahoo rear ending me at speed, and w/o all the work of installing a different tank, sender, gauge, and filling in the stock filler hole!
Bill