How may times do you need more range than you currently have? If it's frequently the case that you need more fuel then perhaps a built-in extra tank is worth it; however if it's rare that you actually need more fuel then fuel cans are a better option. You don't add more weight to your vehicle except when needed, the cans can be carried empty until your last fill before the trajectory for which you need the extra fuel, and can be stored when not in use on a trip.
We've considered adding an extra built-in tank, however so far have actually needed more fuel than we can carry only three times:
South America - once, to cross the SW Bolivian desert, 470 Km travelled, carried fuel for 750 Km
Africa - once, to travel Maun to Kasane, 850 Km travelled, carried fuel for 1000 Km
Australia - once, to cross the Simpson - 610 Km travelled, carried fuel for 870 Km
Everywhere else - never needed.
Our range in all three cases was around 500 Km. For Bolivia, we could have made the crossing with what we had but always want to have a reserve in case of problems. In Botswana and Australia we needed the extra. Fuel cans gave us the reserve needed, however for much of these trips they were empty (for Bolivia we had them available only for the crossing, in Africa we had them throughout the trip, in Australia we had them for most of the trip, giving two away at Normanton, and the rest in NSW, to the Fire Service.
About the only reasons we can see for a built-in extra tank are:
- that cans can be stolen (we lock them onto the roof rack) and the fuel in them can be stolen even when the cans are secured (we use tie-wraps to secure the caps, recognising that these don't provide complete security. If we were really concerned we''d lock the caps).
- when full, cans on the roof rack add to the CoG.
- whether full or not, cans on the roof add some wind resistance.
The negative to a built-in extra tank:
- adds more weight, all the time (obviously less when empty, but still additional weight)
- uses space which could be used for something else (we use the space where we could put an extra built-in tank) for recovery kit, jump leads, and a few odds and ends.
- Need extra fuel lines , wiring, etc.
- costs more.