At 12L/100km that's 800km range ... two extra jerries gets you an extra 335km range for a total of 1135km.
At 15L/100km that's 635km range ... two extra jerries gets you an extra 267km range for a total of 902km.
... and if your doing 10L/100 then a tank & two jerrys will get you 1350km range.
As soon as there's room in the tank for a jerry can, put it in (and carry empty jerry's on the roof) ... get the weight low down asap.
For the price of 2x jerrys I don't think the hassle of a LR tank is worth it UNLESS you are doing major distances constantly ... or are driving a V8 !! 
.... figures above are for a Td5 of course
Kev..
2014 Isuzu MUX LST with fruit
2008 Isuzu D-Max
2015 Kimberley Kamper "Classic"
Gone ... 2004 D2a Td5 Auto "Classic Country" Vienna Green
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