I'm intrigued by this idea, but have the same question as redpoint - why not just half fill the tank?
Is it so you can still "fill the tank" for mpg calculation purposes?
I've never liked the idea of carrying more fuel than needed, and spent a fair while thinking about this.
Eventually (after many overly complicated ideas!) I came up with what I call my "remote fuel tank" basically just two 20l jerry cans that I fill up when I need more fuel. I transfer it to the car 5l at a time, and reset my trip meter when it runs out.
When I'm getting near to the end of my "tank" I drive until the miles to empty indicator hits zero and that's when reset the trip meter. By this point I've got the first 5l of the next fill up in a jerry can in the boot.
Basically I'm always driving with the fuel light on, and I think I worked it out as a 15kg equivalent saving.
Theoretically my mpg readings (pen and paper method) are less accurate, but if I do over (or under) estimate it will correct on the next tank.
Noticeable mpg gains for me (mostly stop start city driving) and I can still use the full capacity of the tank if I need to.
Final thought: supposedly putting a ping pong ball in a gas tank is a good revenge tactic - ping pong gets sucked to the fuel pump and cuts the fuel, but floats up when the engine dies causing an impossible to diagnose fueling issue. Probably an urban legend, but worth considering
Chris
