The book tank holds 25lt. The tank i made for mine is 35lt, it is just a taller version of the book tank. there is a drawing on the forum for it.

intending to make mine from 3mm ally/twin baffles/swirl pot and 60ltr cap by going over the diff as well like an inverted L

That sounds more like it. It would make me a bit nervous embarking on any trip with 25ltrs of fuel. You are probably a bit closer to building your tank than i am, would be interested in your design.
Bob

To heavy?
 

just had a thought. 60 ltrs of fuel is about 42kgs hanging on the rear of the car. Do you think the suspension would cope?

Bob

I am fairly confident it would cope easily as it is a beefy set up and the protech
shocks I have are adjustable +apart from the fact the extra weight for me will be a bonus as my engine will be putting out 200+bhp:D and I will need that extra grip.

i have built a 35l ish tank from 2mm ally that is level with top of the diff frame and sits on a grp frame by ditching d13 i could make it simply longer
i think the grp tube work is strong enough without d13 im 80kg and i can stand on the tip of it and jump up and down with no bending or movement
the hole idea was to keep a full size stowage area in the back and get a bigger tank so touring would be possible :D
if you have a metal back end then you would need to redesign the support as the orignal only has two thin strips of metal and d13 that support the backend

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