Darrell wrote:

Certainly there are a number of ways to do this, but the simplest to me
seems like it'd be to run a couple of (fairly fat) lines, one from the
top of the spare tank into the top of the main tank, and one from bottom
to bottom.  Then the "spare" tank would fill up and drain along with the
"main" tank, and your fuel gauge would still work.  This is more of a
parallel setup than a serial one, though, I don't know how you could do
a serial connection and have the fuel gauge work unless you set it up to
drain the secondary tank first and just lived with the fact that the
fuel gauge stays on 'full' for the first 2/3 of your fill.

It might be easier to do what dual-tank trucks do (at least the ones
with which I'm familiar), although it'd require a bit of electrical
knowhow.  This Ford van that I used to have had two tanks and a switch
under the dash to switch between them.  The switch then would use the
level sender from the selected tank and turn on that tank's fuel pump (I
guess, never really worried about it too much since it Just Worked). Ahh
those were the days, when $50 at the pump actually got you a huge load
of fuel.

-tom!

--