Need - no, Nice to have - yes, Expensive - yes, Worth the price - probably not, as they're usually ridiculously expensive. You can always get something to carry fuel in temporarily. In fact at a couple of places where it's a long ways to the next fuel, locals sit at the side of the road with Coke bottles full of fuel.
My own experience with a 40 litre tank was that it was a nice to have, a real luxury not to have to think about fuel - but you could also get lazy.... We only NEEDED it once, in Nicaragua, when it was a three day wait to get a permit to get in the 3 day line to get the fuel. We ended up riding right through to Costa Rica, and literally coasted into the first gas station and stopped nicely at the pumps, bone dry.

__________________
Grant Johnson
Seek, and ye shall find.
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Inspiring, Informing and Connecting travellers since 1997!
www.HorizonsUnlimited.com

I would say it is more about range and a minimum of 200 miles/320kms is enough in most parts of the world now with another 50miles/80km needed in a few other places.
Bikes are more economical now and those large tanks were needed on airhead BMWs which only did 50 mpg on a good day but some modern medium sized bikes can do 80 mpg.

As mentioned above - its about range, not about how many liters you get into the tank. On a Honda CT 125 Hunter Cub for example with a 1,4 US gallon tank/5,3 liter you will get a range of approx 350 kms. So no need for a 35 liter tank on that bike....

There are several things to consider when it comes to fuel capacity and range:

1. Where are you going to ride? What range do you need? In remote areas where gas stations are scarce, and sometimes even out of fuel? Try to do some research ahead.....

2. What range do you have? Remember that in some countries in the poorer parts of the world fuel sometimes is of lower quality than in the developed parts of the world. For example in Bolivia the fuel is (said to be) 85 octane RON. Which is approx 80 octane AKI as is the north american standards. In Indonesia the standard fuel is 88 octane RON, approx 82 octane AKI. And less octane number - the higher fuel consumption on modern EFI bikes at least. Carburated bikes might be different.

3. I wouldnt had gone anywhere without a fuel range of at least 300 kms!
A fuel range of 400 kms would actually be a minimum fuel range imo for a decent overland bike.
500 km fuel range - now were talkin.
600 km range and above - luxury!

4. As Grant mentioned above - the oversized tanks that are offered are often very highly priced. So that must also be taken into consideration.

5. I rode around the world with two 3,5 liter jerrycans attached to the rear of the side panniers. What a PITA! I have lost count over how many times I needed to reinforce the attachments of those jerrycans, and I lost a couple of those jerrycans too along the road! And that was with designated cans, designated mounts, designated straps etc etc. I wonder how it would work if one used something more amateurish stuff to carry outside gas in?
Imo fuel should be carried in the main gas tank. Carrying fuel outside the tank takes up space for other items such as water, spare parts, clothes, tools etc etc.
And as in my case - outside carried fuel cans tends to fall off. Maybe those rotopax cans work, but they still take up space for other potential stuff you would like to carry.

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In the end everything will be fine. If its not fine its not the end....

I agree with this 100% but would like to add that when you go to places where those extra 80km just maybe might be required, that you get in the habit of topping up your main tank often (30-50%). The fuel availability of the last 1000 km may be a poor predictor of fuel availability for the next 100km.

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That's a fair assessment.. there isn't that many places where you need 250mi+ range. Still IMO 701LR got it right. For one, even if gas available there could be issues with quality, and on some remote vistas you not only need fuel to get there, but also need to carry extra to be able to turn back at any point.

That was really lucky as Susan would have been so hacked off having to push you sat on the bike to the next fuel station.

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You will have to do without pocket handkerchiefs, and a great many other things, before we reach our journey's end, Bilbo Baggins. You were born to the rolling hills and little rivers of the Shire, but home is now behind you. The world is ahead.

But imagine what a great story you would have to tell if you'd been stuck in Nicaragua with no fuel!

I bought my 790 with the specific intention of riding through Siberia and the Stans, and my desktop research said there were stretches of over 400 kms with no fuel. The 790 is pretty economical and with a 20L tank will easily beat 400 kms (I think I did about 450 kms on one tankful in Italy in 2019). I don't really want to fall about with auxiliary tanks or huge top heavy monsters so the stock range should be adequate. That said with belt and braces in mind I will also be taking a 6.7 L Rotopax

As someone who is currently planning a RTW trip, the size of the gas tank is an important consideration for me. My goal is to have a min 500 KM range - not because there's not enough gas stations, but because of numerous mentioned accounts above. Anybody who's ridden distance has experienced that nagging insecurity of being in parts unknown and not knowing if you'll make it to the next gas station - and if there will be gas there. Then mentally doing the calculations to see how much further you've got before running out - and looking at the sky and seeing the weather locking in. Or waking up in a camp site far off the beaten track hoping that you make it to the next station....

Having to stop vs deciding to stop are two different things, and I want that flexibility. And we've all also experienced that gas station where we wished we coulda kept going....

One thing I haven't noticed being addressed above: topping off from external fuel tanks--whether coke bottles, plastic fuel cans with spouts, or bladders--is not always a simple thing. I learned this to my dismay on one of the very few occasions when my ~20 liter tank wasn't sufficient (a series of towns without fuel in Patagonia); decanting in 100+ kph winds left me, the bike, and the surrounding ground covered with gasoline even though I'd pulled in behind a building for shielding.

On another occasion (dusk combined with miscalculations driving over a remote 15,000 ft pass in the Andes) I ran out of gas at a low spot in the road. Of course I was carrying extra fuel, but the real problem was that I'd busted my clutch cable and didn't have a spare because I'd previously busted my spare. Getting the bike underway was quite a project in itself, but at least I didn't have to spend the night out in the cold.

On both occasions better planning would have saved me some trouble, but needing to plan better can be a real source of stress.

yes if your going round the world ......... big tank ........... but for 99% of us a standard tank is ok ?

Absolutely!! Even for RTW, it's highly debatable. You will only need it a couple of times, is it worth the price? Is the ease of not having to pay attention to your fuel worth the price? And remember, even with the big tank, you CAN run out because you forgot to fill up because you do it so seldom!!

__________________
Grant Johnson
Seek, and ye shall find.
------------------------
Inspiring, Informing and Connecting travellers since 1997!
www.HorizonsUnlimited.com

We make the SAFARI TANKS here in Australia, most of the time you wouldn't need the capacity (normal roads) but if your going 'bush' its pretty much essential or if you're Dakar type racing (flat out).

Mezo.

australia is a BIG place ! but most of the popultion live where petrol stations are in most towns!

Ha ha, been there, done that. With some of the opaque big tanks you need a torch to see how much is down there. If you don't have one handy the workaround is to shake it, listen to the sloshing sound and make a guess. Sometimes you get the guess wrong ...

A lot of the plastic ones are translucent so you can see the fuel level inside. I've marked mine up on the outside in 5L amounts with a black marker and that helps. The alternative is to plumb in some clear plastic tubing at the bottom - either via drilling a hole and a spigot or a T from the petrol tap, and run it up the outside so you can see the level in the tube (early Japanese road bikes used to do it as a crude fuel gauge).