gravity feed auxiliary fuel tank
 

I was trying to get info on if anybody has attempted a trunk mounted gravity feed fuel tank. i get around 220 miles on a full tank of e85, limited stations around me. I was wanting to add a 10 gallon tank in the trunk. I have only seen this with guys on trucks mounting the tanks in their bed.

Not that I actually recommend doing this, but they do make fuel cells that fit in place of the spare wheel/tire. That's probably your best bet since putting something in the trunk would likely prevent you from getting to the spare anyway.

It's a pressurized system.
With a vent outside for the gravity to work, you'll likely get a CEL into limp mode.
Depending how high, it may drain into the EPA carbon evaporation canister and ruin that too.

A couple race fuel jugs in the back would be simpler, but you need to get out to unstrap them and top off.